noindex, nofollow, rel-canonical for non-stable docs ?

[Carreau]

📚 Documentation

After the beautiful work from @melissawm on napari/napari#4635, it would be good to have the ability to update the older documentation pages with "noindex", "nofollow" and rel="canonical". This should make sure search engine do not index old docs, and give the proper weight to the right pages.

In particular I think that all the rel='canonical' should point to "stable".

I don't have particular thoughts on whether /dev/ should be no-index/no-follow.

I believe a good way of doing that would be to inject a placeholder in the theme that is easy to search/replace.