Add keyboard shortcuts for repository file tree browser to shortcuts menu

Follow-up from #541107 (closed)

Add keyboard shortcuts for the repository file tree browser to the keyboard shortcuts menu.

Currently, only the up/down arrow keys are documented. We need to add documentation for all navigation shortcuts:

Keyboard shortcuts to document:

• Up/Down Arrow - Navigate between tree items
• Right Arrow - Expand directory / Move to first child
• Left Arrow - Collapse directory / Move to parent
• Enter/Space - Select items
• * - Expand all directories on the same level as the selected one

Related discussion: #541107 (comment 2909722700)

This may require input from Technical Writing team on how to present the multiple use cases for left/right navigation (e.g., different behavior on closed vs open directories, files vs folders, etc.). Can leverage the keyboard shortcut sections added in MR !207244.