docs(grid): make grid cards responsive for phone and tablet

why: Grid cards used single-value column counts ({grid} 2, {grid} 3)
which don't collapse on small screens, squeezing content uncomfortably.
what:
- 2-col grids: {grid} 1 1 2 2 (stack on phone/tablet, 2-col on desktop)
- 3-col grids: {grid} 1 2 3 3 (1-col phone, 2-col tablet, 3-col desktop)
- Gutters: 2 2 3 3 (tighter spacing on mobile)
- Uses sphinx-design's responsive breakpoint syntax (xs sm md lg)

Author: tony

docs/api/index.md

@@ -12,8 +12,8 @@ Server -> Session -> Window -> Pane
 
 ## Core Objects
 
-::::{grid} 2
-:gutter: 3
+::::{grid} 1 1 2 2
+:gutter: 2 2 3 3
 
 :::{grid-item-card} Server
 :link: libtmux.server
@@ -43,8 +43,8 @@ Terminal instance. Send keys and capture output.
 
 ## Supporting Modules
 
-::::{grid} 3
-:gutter: 3
+::::{grid} 1 2 3 3
+:gutter: 2 2 3 3
 
 :::{grid-item-card} Common
 :link: libtmux.common

docs/index.md

@@ -5,8 +5,8 @@
 Typed Python API for [tmux](https://github.com/tmux/tmux). Control
 servers, sessions, windows, and panes as Python objects.
 
-::::{grid} 2
-:gutter: 3
+::::{grid} 1 1 2 2
+:gutter: 2 2 3 3
 
 :::{grid-item-card} Quickstart
 :link: quickstart

docs/project/index.md

@@ -4,8 +4,8 @@
 
 Information for contributors and maintainers.
 
-::::{grid} 2
-:gutter: 3
+::::{grid} 1 1 2 2
+:gutter: 2 2 3 3
 
 :::{grid-item-card} Contributing
 :link: contributing

docs/topics/index.md

@@ -2,8 +2,8 @@
 
 Explore libtmux's core functionalities and underlying principles at a high level, while providing essential context and detailed explanations to help you understand its design and usage.
 
-::::{grid} 2
-:gutter: 3
+::::{grid} 1 1 2 2
+:gutter: 2 2 3 3
 
 :::{grid-item-card} Architecture
 :link: architecture