[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.