[Website] Link to other `apache/arrow-___` repos

[ianmcook]

On the Arrow website and in the main Sphinx docs, the prominent GitHub links all point to the monorepo. With more of the Arrow implementations now moving outside of the monorepo (Go most recently, Java planned next), we need to help website and docs visitors see that these other apache/arrow-___ repos exist. Find a good way to do this. For example, replace the GitHub link with a drop-down menu that provides links to all the Arrow repos.

[ianmcook]

@thisisnic this is the issue for the topic we were discussing today.

The idea that I floated was this:

On the top left nav of the Sphinx site, rename the Implementations drop-down menu to Docs, and add a Source drop-down menu linking to all the separate repos or subdirectories of the main repo where the source for different project components are maintained. These links can be copied from the README. It would look like this when the menus are collapsed:

We could also change the top nav in the arrow-site repo to match this, renaming Documentation to Docs and adding a Source drop-down:

[thisisnic]

Thanks for summarising this @ianmcook! PR for arrow-site is here: #653