Add navigation between forks in the rendered documentation

[SamWilsn]

Would be nice to be able to swap between forks without having to navigate all the way back up the directory tree. We should re-implement the feature we had in the old Sphinx plugin to add a drop down on fork pages.

Option A: Left Sidebar

Option B: Top

Option C: Right Sidebar

Has the advantage of including the diffs as well.

[shrirajpawar4]

@SamWilsn I have a #2799 open. can i take this issue and fix this stacking on that PR?

[SamWilsn]

If you'd like! You can dive right in, or if you'd like, sketch out your approach first here.