Add Previous Releases feature to Furo

[Eric-Arellano]

Closes #381.

The implementation mostly reuses our code for translations (#380).

See #355 for an explanation of why we diverge Furo's standard approach of the top-level entry being clickable and going to a dedicated HTML page. Instead, we have the entry look like a sidebar entry, and still have the entire div be clickable.

Reduces padding for translations

This reduces padding for the translations header, which aligns it with the rest of the sidebar entries. I'm not sure why I had set up so much padding before.

@SooluThomas and I thought this is a cleaner design, and that it takes less screen real estate. But we can add back the padding if people want.

Tweaks example_docs table of contents

To make our example docs more clear, this adds a new Special Elements section. On Pytorch, that results in the ToC looking like this:

[javabster]

overall looks good, just one small thing: now that we have reduced the size of languages padding it now looks like the searchbar is a bit too squished up to the top. I would recommend adjusting the spacing so there is an equal amount of padding above and below the search bar, so either increase the padding between the languages and searchbar or decrease the padding between searchbar and "documentation home"

[javabster]

LGTM 🚀