Draft: Add shortcuts table to editor extensions docs

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What does this MR do?

Adds/updates a shortcuts table in doc/editor_extensions/shortcuts.md with clean descriptions.

Why?

Key name cleanup requires a full shortcut list (ref #YOUR_ISSUE_ID).

Screenshots (if applicable)

How to setup/test

• Run gdk start and navigate to /help/editor_extensions/shortcuts.
• Verify table renders correctly.