documentation: support versioned docs

[protobits]

As previously discussed, the sphinx documentation on the nuttx website should offer a version selector to allow reading docs for master ("latest") or for past releases (starting from following one).

While there's already some basic skeleton to allow for this (I added an HTML template that appears on the sidebar), we need add some code to conf.py so that it picks up the list of releases we would like to offer. It could either be based on running some git commands from that script or it could read an environment variable set by the workflow with the same information.

This would simply fill the sidebar with links like nuttx.apache.org/docs/. A subsequent PR should also build the documentation for each release as well (probably by adding a trigger to act on pushes to release branches or tags, depending on how we want to deal with that).

Note that we cannot simply add a hardcoded list to conf.py since this would mean that the rendered version of docs for each release would only "know" about the past releases at that time. Thus, this information needs to be generated during build so that all versions of the documentation have links to all other offered versions.