Document custom MacBook trackpad gestures

[lupinsansei]

Summary

• Document optional three-finger swipe up/down gestures for fullscreen and restoring the focused window.
• Clarify that the existing horizontal three-finger gesture changes workspaces.

This gives MacBook users a small, useful customization recipe while keeping the behavior explicitly optional.

Testing

• git diff --check
• Reviewed the rendered Markdown structure and Lua example.