docs: user can select version (instead of HEAD/nightly)

[dundargoc]

There have been some instances where users are confused why the documentation in neovim.io doesn't work only to realize that they don't use nightly. I have a few ideas on how we could tackle this:

• Allow users to choose the version for the documentation. I think a good example of this is cmake (crazy, right?), where the documentation version is prominently shown at the top in an easy place.

• Use the latest stable in the documentation instead of nightly. This is probably easier to implement. A problem might be that nightly users might start complaining they can't find docs for nightly features.

Open to other suggestions.

[justinmk]

Use the latest stable in the documentation instead of nightly.

Not in favor. The website docs allow us to make hints and other docs available via the website search (and search engines).

Allow users to choose the version for the documentation.

This would be ideal. But the HEAD docs should stay at the current paths, because:

• avoids breaking URLs
• avoids needing to update our Algolia config

The version-specific docs could live at e.g. https://neovim.io/doc/user/v0.10/.