docs(redesign): restructure documentation to Library Skeleton pattern

why: The documentation had accumulated organically — test helpers and
pytest plugin floated as top-level orphans, API reference files used
ambiguous names (servers.md vs libtmux.server.md), internal APIs weren't
separated from public ones, and the landing page dumped the full README
with no navigation affordance. This restructure follows the Python
Documentation Skeletons spec where api/ is the primary reference surface
for a library package.

what:

Structure:
- Rename api/ reference files to libtmux.<module>.md format (10 files)
- Move test-helpers/ and pytest-plugin/ under api/ as auxiliary public
  API surfaces
- Restructure internals/ with api/ subdirectory using
  libtmux._internal.*.md naming
- Create project/ directory (contributing, code-style, releasing)
- Move developing.md to project/contributing.md
- Fold about.md content (ID prefixes, naming conventions) into
  topics/architecture.md

New pages:
- api/public-api.md — stability contract, pre-1.0 semver policy,
  deprecation process
- api/compatibility.md — Python/tmux/platform support matrix
- api/deprecations.md — active deprecation tracker
- topics/architecture.md — module hierarchy, data flow, internal
  identifiers (merged from about.md)
- topics/configuration.md — env vars, format handling
- topics/design-decisions.md — ORM rationale, format strings, neo.py
- topics/public-vs-internal.md — boundary philosophy, promotion path
- project/code-style.md — ruff, mypy, NumPy docstrings
- project/releasing.md — git tags, OIDC trusted publishing

Landing page:
- Compose standalone homepage (no README.md include)
- One-sentence intro, 2x2 grid cards, 2-command install with pin tip,
  6-line code snippet with hierarchy diagram, pytest fixture example

Section indexes:
- api/index.md: 2x2 Core Objects + 3x2 Supporting Modules card grids
- topics/index.md: 2x4 card grid for substantive topic pages
- project/index.md: 2x2 card grid for contributor pages

Navigation:
- Sidebar: Quickstart, Topics, API Reference, Internals, Project,
  Changelog, Migration, Glossary
- 22 redirects for all renamed/moved files
- README.md API URLs updated to new libtmux.<module>.html paths

Dependencies:
- Add sphinx-design to docs and dev dependency groups
- Annotate all doc dependencies with documentation site URLs

conf.py:
- Add sphinx_design extension
- Add myst_heading_anchors = 4

Author: tony

README.md

@@ -230,12 +230,12 @@ Session($... ...)
 
 | libtmux object | tmux concept                | Notes                          |
 |----------------|-----------------------------|--------------------------------|
-| [`Server`](https://libtmux.git-pull.com/api/servers.html) | tmux server / socket | Entry point; owns sessions |
-| [`Session`](https://libtmux.git-pull.com/api/sessions.html) | tmux session (`$0`, `$1`,...) | Owns windows |
-| [`Window`](https://libtmux.git-pull.com/api/windows.html) | tmux window (`@1`, `@2`,...) | Owns panes |
-| [`Pane`](https://libtmux.git-pull.com/api/panes.html) | tmux pane (`%1`, `%2`,...) | Where commands run |
+| [`Server`](https://libtmux.git-pull.com/api/libtmux.server.html) | tmux server / socket | Entry point; owns sessions |
+| [`Session`](https://libtmux.git-pull.com/api/libtmux.session.html) | tmux session (`$0`, `$1`,...) | Owns windows |
+| [`Window`](https://libtmux.git-pull.com/api/libtmux.window.html) | tmux window (`@1`, `@2`,...) | Owns panes |
+| [`Pane`](https://libtmux.git-pull.com/api/libtmux.pane.html) | tmux pane (`%1`, `%2`,...) | Where commands run |
 
-Also available: [`Options`](https://libtmux.git-pull.com/api/options.html) and [`Hooks`](https://libtmux.git-pull.com/api/hooks.html) abstractions for tmux configuration.
+Also available: [`Options`](https://libtmux.git-pull.com/api/libtmux.options.html) and [`Hooks`](https://libtmux.git-pull.com/api/libtmux.hooks.html) abstractions for tmux configuration.
 
 Collections are live and queryable:
 

docs/about.md

@@ -0,0 +1,29 @@
+# Compatibility
+
+## Python
+
+- **Minimum**: Python 3.10
+- **Tested**: Python 3.10, 3.11, 3.12, 3.13
+- **Maximum**: Python < 4.0
+
+## tmux
+
+- **Minimum**: tmux 3.2a
+- **Tested**: latest stable tmux release
+- libtmux uses tmux's format system and control mode -- older tmux versions
+  may lack required format variables
+
+## Platforms
+
+| Platform | Status |
+|----------|--------|
+| Linux | Fully supported |
+| macOS | Fully supported |
+| WSL / WSL2 | Supported (tmux runs inside WSL) |
+| Windows (native) | Not supported (tmux does not run natively on Windows) |
+
+## Known Limitations
+
+- tmux must be running and accessible via the default socket or a specified socket
+- Some operations require the tmux server to have at least one session
+- Format string availability depends on tmux version

docs/api/compatibility.md

@@ -0,0 +1,16 @@
+# Deprecations
+
+Active deprecations with timeline and migration paths.
+
+## Active Deprecations
+
+<!-- Pull active deprecations from CHANGES when they exist -->
+
+No active deprecations at this time.
+
+See [history](../history.md) for past changes and the
+[migration guide](../migration.md) for upgrading between versions.
+
+## Deprecation Policy
+
+See [Public API -- Deprecation Process](public-api.md#deprecation-process).

docs/api/deprecations.md

@@ -4,16 +4,103 @@
 
 # API Reference
 
+libtmux's public API mirrors tmux's object hierarchy:
+
+```
+Server -> Session -> Window -> Pane
+```
+
+## Core Objects
+
+::::{grid} 2
+:gutter: 3
+
+:::{grid-item-card} Server
+:link: libtmux.server
+:link-type: doc
+Entry point. Manages sessions and executes raw tmux commands.
+:::
+
+:::{grid-item-card} Session
+:link: libtmux.session
+:link-type: doc
+Manages windows within a tmux session.
+:::
+
+:::{grid-item-card} Window
+:link: libtmux.window
+:link-type: doc
+Manages panes, layouts, and window operations.
+:::
+
+:::{grid-item-card} Pane
+:link: libtmux.pane
+:link-type: doc
+Terminal instance. Send keys and capture output.
+:::
+
+::::
+
+## Supporting Modules
+
+::::{grid} 3
+:gutter: 3
+
+:::{grid-item-card} Common
+:link: libtmux.common
+:link-type: doc
+Base classes and command execution.
+:::
+
+:::{grid-item-card} Neo
+:link: libtmux.neo
+:link-type: doc
+Dataclass-based query interface.
+:::
+
+:::{grid-item-card} Options
+:link: libtmux.options
+:link-type: doc
+tmux option get/set.
+:::
+
+:::{grid-item-card} Hooks
+:link: libtmux.hooks
+:link-type: doc
+tmux hook management.
+:::
+
+:::{grid-item-card} Constants
+:link: libtmux.constants
+:link-type: doc
+Format strings and constants.
+:::
+
+:::{grid-item-card} Exceptions
+:link: libtmux.exc
+:link-type: doc
+Exception hierarchy.
+:::
+
+::::
+
+## Test Utilities
+
+If you're testing code that uses libtmux, see the test helpers and pytest plugin:
+
+```{toctree}
+:maxdepth: 1
+
+test-helpers/index
+pytest-plugin/index
+```
+
+## API Contract
+
 ```{toctree}
+:maxdepth: 1
 
-properties
-servers
-sessions
-windows
-panes
-options
-hooks
-constants
-common
-exceptions
+public-api
+compatibility
+deprecations
 ```