Add documentation for new flexible menu item API (al_menu_items_custom_items)

[Copilot]

This PR adds comprehensive documentation for the new flexible menu item API introduced in PR #726, which allows interview authors to add custom menu items to the dropdown/hamburger menu in Assembly Line interviews.

What's New

The new al_menu_items_custom_items variable enables developers to:

• Add custom menu items to interview navigation menus
• Display custom items before default Assembly Line menu items (Start over, Exit, etc.)
• Create both static and dynamic menu items with show/hide logic
• Use standard Docassemble menu item format with url, label, and optional hidden keys

Documentation Added

Basic Static Menu Items

Simple configuration using a data block for static menu items:

---
variable name: al_menu_items_custom_items
data:
  - url: https://example.com/help
    label: Help and Support
  - url: https://example.com/about
    label: About this form

Dynamic Menu Items with Show/Hide Logic

Advanced configuration using data from code with conditional visibility:

---
reconsider: True
variable name: al_menu_items_custom_items
data from code:
  - url: |
      "https://example.com/help"
    label: |
       "Help and Support"
  - url: |
      url_action('save_progress')
    label: |
       "Save my progress"
    hidden: not user_logged_in() or (_internal.get('steps') < 2)

Best Practices Included

• Using reconsider: True for dynamic menu items
• Avoiding undefined variable triggers with showifdef(), defined(), and hasattr()
• Progressive disclosure patterns with _internal.get('steps') < 2
• Context-sensitive help menu examples

The documentation is integrated into the existing "Special variables" page under the "Run-time options" section, maintaining consistency with the existing documentation structure.


Fixes #369.

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