docs: Add a "Keyboard Shortcuts" reference

Summary The labeling canvas has shortcuts (tool selection, navigation, save) that aren't documented in one place. Create a reference table.

Where

• New file docs/keyboard-shortcuts.md, linked from the README.
• Source of truth: search the frontend for keybindings in apps/studio/src/features/studio and apps/studio/src/shared (keydown, hotkey, useKeyboard, or a shortcuts map).

Steps

1. Branch: git checkout -b docs/keyboard-shortcuts.
2. Grep the codebase for keyboard handlers to compile the real, current list (don't invent shortcuts).
3. Create docs/keyboard-shortcuts.md with a table: Action | Shortcut (Win/Linux) | Shortcut (macOS).
4. Link it from README.md.
5. Run yarn lint; commit docs: add keyboard shortcuts reference and open a PR.

Acceptance criteria

• The doc lists shortcuts that actually exist in the code.
• It's linked from the README.

Good first issue — comment here if you'd like to take it.