feat(lsp): experimental/localDocs support

[theammir]

I opened #805 yesterday. Turns out, it's really not rocket science to implement.

I am worried about HTML URIs potentially being handled by a text editor, though.
Also, we're implicitly relying on outdated documentation for local types, as there's nothing that rebuilds it on save. It is still better than a 404 page, I think.
And I sure hope all 4 opening strategies can handle file:// identifiers.

I couldn't properly run luarocks tests.

[github-actions]

Review Checklist

Does this PR follow the Contribution Guidelines? Following is a partial checklist:

Proper conventional commit scoping:

• For example, fix(lsp): some lsp-related bugfix

• Pull request title has the appropriate conventional commit prefix.

If applicable:

• Tested
  • Tests have been added.
  • Tested manually (Steps to reproduce in PR description).
• Updated documentation.

[mrcjkb]

Thanks 🙏

Lgtm

[mrcjkb]

Will have to run the docgen hook when I'm on my laptop.