Update generated proto reference doc to .mdx

[danbarr]

Summary

Renames the proto.md generated reference doc to proto.mdx.

Future proofing: The file has inline JSX. Docusaurus currently renders all .md and .mdx with the MDX compiler so this works, but the next major version will change this (.md will be compiled with CommonMark instead; reference).

Change Type

Mark the type of change your PR introduces:

• Bug fix (resolves an issue without affecting existing features)
• Feature (adds new functionality without breaking changes)
• Breaking change (may impact existing functionalities or require documentation updates)
• Documentation (updates or additions to documentation)
• Refactoring or test improvements (no bug fixes or new functionality)

Testing

Ran make gen to confirm the .mdx file is generated as expected and the docs site builds.

Review Checklist:

• Reviewed my own code for quality and clarity.
• Added comments to complex or tricky code sections.
• Updated any affected documentation.
• (N/A) Included tests that validate the fix or feature.
• (N/A) Checked that related changes are merged.