Astro 5 + collection loaders + legacy collections support

[HiDeoo]

Description

This PR is a draft adding Astro 5 support to Starlight, adding collection loaders for our content collections and support for legacy collections.

The first goal was to get everything working with Astro 5. There is room for improvements and potential refactoring, but the goal was to get everything in place first to get a better overview of what needs to be done and later iterate on it.

The MDX dependency case is temporarily workaround by making it a peer dependency of Starlight and installing it manually everywhere it's needed until we have a better solution (in discussion).

Note that the collection locations are not yet user-configurable.

Remaining tasks

• Ensure all // TODO(HiDeoo) comments are addressed and removed
• Figure out if legacy collections support should be documented more (it's only a sentence + link in the manual setup page now)
• Peer dependency bump for @astrojs/starlight in some packages (see changesets for the one that needs an update)
• Add changesets
• Merge with Lunaria directives to ignore the changes to all code.mdx files
  • @lunaria-ignore:src/content/docs/**/code.mdx;src/content/docs/code.mdx;