Add menu support

Summary

Add menu support to the Content First module, allowing site menus to be viewed and exported as Markdown for use in AI workflows, documentation tools, and Obsidian-style knowledge bases.

Problem / motivation

The Content First module currently supports nodes only. Site menus represent important structural content — navigation hierarchies, link labels, descriptions — that AI assistants and content editors need access to in plain-text format. There is no way to inspect or export a menu as Markdown through the existing UI or Drush command.

Proposed resolution

Add a Content First tab to the menu administration interface at /admin/structure/menu/manage/{menu}/content-first that renders the full menu tree as a Markdown list, with Copy and Download buttons.
Extend the content-first:export Drush command to support menus via --entity=menu, exporting one menu-{langcode}-{name}.md file per language per menu.
Change the default value of --entity from node to all, so a bare drush content-first:export exports all supported entity types.
Add --rewrite-links and --assets-base-url options to convert internal paths to local filenames and make asset URLs absolute, enabling full Obsidian vault import.

UI changes

A new Content First local task tab appears on every menu edit page (requires the Administer menus and menu links permission). The tab renders the menu tree as a fenced Markdown block with:

Nested list items following standard Markdown indentation.
Title for linked items; plain Title for items.
Optional ": description" appended when a description is set.
"(disabled)" suffix on disabled menu links.
Copy-to-clipboard and Download-as-file buttons above the content block.

Drush command changes

New --entity values: node (nodes only), menu (menus only), all (both, new default).

New and updated options:

--bundles — for menus, accepts menu machine names (e.g. main,footer).
--status — for menus: 1 = enabled items only, 0 = disabled items only, omit = all (disabled items marked with "(disabled)").
--rewrite-links — converts internal page link paths to local filenames matching the exported node filename pattern (e.g. /en/about → en_about). Images and document files are excluded.
--assets-base-url= — makes internal asset URLs (images and documents such as PDF, DOCX, ZIP) absolute by prepending the given domain. Can be used independently of --rewrite-links.
Output filename convention for menus: menu-{langcode}-{menu-machine-name}.md

Usage examples

drush content-first:export
Export everything (nodes + menus, all languages).

drush content-first:export --entity=node
Export nodes only.

drush content-first:export --entity=menu
Export menus only.

drush content-first:export --entity=menu --bundles=main,footer
Export specific menus.

drush content-first:export --rewrite-links --assets-base-url=https://example.com
Full Obsidian import: page links as filenames, assets as absolute URLs.

Technical notes

Menu tree loading uses MenuLinkTreeInterface::load() with MenuTreeParameters::setMaxDepth(9) and the default sort manipulator.
Multilingual titles and descriptions are resolved by loading the underlying menu_link_content entity and calling getTranslation($langcode), avoiding any global language context switch.
The menu route uses _permission: 'administer menu' and the entity:menu parameter converter, consistent with all other routes under entity.menu.*.
Asset detection for --assets-base-url is extension-based (pathinfo PATHINFO_EXTENSION), covering common image and document formats.