docs: Write keyboard shortcuts reference

[ggfevans]

Summary

Create keyboard shortcuts reference page.

Migrated from: RackulaLives/www#18

Content Outline

Complete Shortcut Table

• Navigation shortcuts
• Editing shortcuts
• View shortcuts
• File shortcuts

Power User Tips

• Modifier key combinations
• Quick workflows

Acceptance Criteria

• Complete table of all shortcuts
• Grouped by function
• Searchable format