Add symlink "stable" pointing to 1.18 docs

[rossbar]

With the inclusion of this symlink, users pointing to the URL https://numpy.org/doc/stable/ should see the index.html for the most recent documentation (the link currently points to the 1.18/ directory).

I enabled GitHub Pages on my own fork and verified that the link works as expected: see https://rossbar.github.io/doc/stable

Let me know if there are any details missing re: how the docs are hosted.

[mattip]

Did we also want a "latest" link pointing to "devdocs" (or the other way around: push to "latest" and create a link to it from "devdocs", and in a while drop "devdocs")?

[rossbar]

Good question, I'm not 100% sure. The only thing that I had written in my notes from previous meetings was about adding /stable/. I'll bring it up at the meeting today.

[mattip]

Thanks @rossbar