Support for rehosted documentation

[mortenpi]

Allow some docs to be marked as "rehosted". This would:

1. Add a noindex tag to all of the pages, so that the rehosted pages wouldn't show up on Google etc.
2. Add a banner indicating that canonical URL.

X-ref: SciML/SciMLDocs#151 (comment)

[asinghvi17]

Can't we just explicitly link to the docs, and merge the search index of those docs into our own somehow? Assuming they are all Documenter this should work fine.

(Issues will arise of course with Vitepress, but then that doesn't work with MultiDocumenter anyway at this point)

[pfitzseb]

Yes.