CPS "releases" should be tagged

[mwoehlke]

Right now, there is no easy way to determine what CPS "was" as of a certain version. (This is also a bit fuzzy, since we haven't always remembered to bump version numbers, and not all changes to the repo modify the version number.)

Still, it would be good if we had some sort of tag system. For a first pass, every commit that changes the version number should get a tag that matches the new version number.

Moreover, this seems like a fine candidate for something that can be automated in a GHA.

Community contributions for such a feature/workflow/GHA would be welcomed. (Ideally it should scan from the newest tag to HEAD and tag all version changes identified, such that the first run can bootstrap the backlog of non-existing tags. To facilitate this, especially as 0.5 was the first version to have a conf.py, that version has now been tagged.)