docs: add Dock docs and gallery example (#2782)

## Summary

- add English and Chinese gpui-component Dock guides directly after Root
View
- add a styled Dock story to the gpui-component Gallery, including
draggable tabs and a collapsible bottom dock
- restore Gallery WASM builds for local docs and GitHub Pages deployment
- keep the final split slot flexible so Dock continues filling the
viewport after drag operations

## Test Plan

- `cargo test -p gpui-base dock:: --quiet`
- `cargo check -p gpui-component-story-web`
- `cd website && bun run build`
- `make -C crates/story-web build`
- verified `/gpui-component/gallery/?story=Dock` and Gallery assets are
served by the local docs server

Co-authored-by: Codex <codex@openai.com>