Make stellar-docs show documentation for multiple versions

[fnando]

What problem does your feature solve?

Right now, we can't view the upcoming release's docs without removing the stable docs.

What would you like to see?

To fix that, we should do something like https://developers.stellar.org/platforms/anchor-platform/ and show a version selector.

For us, it only makes sense to have 2-3 versions at a time:

1. Stable (the latest stable release)
2. Next (the next release, i.e. we have a RC version that's bigger than the stable release)
3. Dev (the main branch)

What alternatives are there?

None

[Ifropc]

cc @ElliotFriend