docs(api[sidebar]): restore core module pages to sidebar navigation

why: The Library Skeleton redesign replaced toctree entries with
sphinx-design grid cards for core modules. Grid cards are purely
visual links — Sphinx only registers pages in the nav tree via
toctree directives, so the modules became orphaned (accessible by
URL but absent from the sidebar).
what:
- Add hidden toctree in docs/api/index.md listing all 10 core and
  supporting modules (server, session, window, pane, common, neo,
  options, hooks, constants, exc)
- :hidden: preserves grid cards as visual entry point while
  restoring sidebar navigation

Author: tony

docs/api/index.md

@@ -84,6 +84,22 @@ Exception hierarchy.
 
 ::::
 
+```{toctree}
+:hidden:
+:maxdepth: 1
+
+libtmux.server
+libtmux.session
+libtmux.window
+libtmux.pane
+libtmux.common
+libtmux.neo
+libtmux.options
+libtmux.hooks
+libtmux.constants
+libtmux.exc
+```
+
 ## Test Utilities
 
 If you're testing code that uses libtmux, see the test helpers and pytest plugin: