Version selection dropdown

[enpaul]

Hello, thanks for all your work on this great theme! I was wondering whether adding a version dropdown similar to the one the sphinx-material theme has would be an accepted feature. I'm working on standardizing some multi-project documentation at work and this is the only feature we need that furo is missing.

If this feature is something you'd be open to then I could work on implementing it.

Thanks again!

[pradyunsg]

This has been previously discussed in #74 (from before discussions were enabled on this project).