Prior versions of the docs should link to more recent versions of the docs

[Eric-Arellano]

It's easy to look at old versions of the docs without realizing there are newer versions.

Our previous versions sidebar only shows prior versions, not new ones:

Fixing this will be tricky because we currently don't rebuild the docs for prior versions. I suspect we don't want to, such as risk of bit rot meaning that the docs rebuild will fail. I'm curious if we can dynamically pull the version info rather than hardcoding it. For example, have a simple API that all versions of the docs can hit to determine the versions to show.

Ideally, we can also have a warning when you are not on the latest version of the docs.