Add an Extend page with a community plugin registry

[mdo]

• Add an Extend page as a new top-level navbar entry beside Examples, driven by a community plugin registry in site/data/extensions.yml and seeded with 26 plugins.
• Let contributors add a plugin by appending one validated YAML block; an extensions Zod schema in site/src/libs/data.ts fails the docs build on a missing or malformed field, and the "Add your plugin" instructions live in a dialog on the page.
• Filter the registry by category and Bootstrap major version from dropdowns built into the table headers, alongside a name/description search box; the v6 option stays disabled until an entry supports it.
• Restyle the SingleLayout header with roomier spacing and vertical centering, now shared by Extend and Examples.

Follow-up to #42770.

[coliff]

PREVIEW: https://deploy-preview-42860--twbs-bootstrap.netlify.app/docs/6.0/extend/

Neat idea and I'm in favour of adding this.

• As the URL has the version in it I'd want/expect the only add-ons/plugins here to be v6 ones (at least by default in the dropdown?).
• I think a version of this page could be backported to the v5 site.
• I don't think v4 plugins need any mention at all since it's been unsupported for so long.
• Would be neat if you could click on Plugin table header to sort alphabetically (or have them alphabetical by default).
• The v5 v4 badges don't have good contrast.
• Maybe at the bottom a link to Explore more - https://github.com/topics/bootstrap ?