docs(faq): add undo delete, timezone override, and keyboard shortcuts

[devin-ai-integration]

Summary

Adds documentation for recently implemented features that were missing from the FAQ:

1. Undo delete sessions - Documents the 5-second undo window that appears as a toast when deleting a session (implemented in add undo delete toast and state management #3557)
2. Timezone override - Documents the manual timezone setting in Settings > General for the sidebar timeline (implemented in manually configure timezone in settings #3552)
3. Cmd+Z keyboard shortcut - Added new "Editing" section to keyboard shortcuts documenting that Cmd+Z also restores recently deleted notes

Review & Testing Checklist for Human

• Verify the undo delete toast appears when deleting a session and the 5-second window is accurate
• Confirm Cmd+Z restores a recently deleted session (within the undo window)
• Confirm the timezone setting is located at Settings > General as documented
• Check that selecting "System" in the timezone dropdown reverts to automatic detection

Notes

Link to Devin run: https://app.devin.ai/sessions/6c97567f265547cdb8f9381827b85c7c
Requested by: unknown ()

[devin-ai-integration]

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[devin-ai-integration]

Closing outdated docs update PR - consolidating into stacked PR workflow