docs: switch to just-the-docs theme

[marquiz]

Replace the unmantained jekyll-rtd-theme

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[marquiz]

Quickly hacked together a version menu in the top-bar/header. No styling for example. Check how it looks. A preview in netlify and in my personal repo (you can see that the version menu actually works).

We'd need to update the old versions under gh-pages, too (and probably just drop most of them). That can be done in a separate PR.

/assign @ArangoGutierrez
/hold

[marquiz]

We could also try https://github.com/JV-conseil/jekyll-theme-read-the-docs