Make release tarballs hermetically in GitHub Actions

[dmnks]

Investigate the possibilities of using GH's built-in feature for release tarballs based on git tags, ideally deploying them (also) to our FTP servers at rpm.org. We could leverage GitHub Actions to automate this.

We would've done this sooner if it weren't for the need to ship some built artifacts (documentation) in the release tarballs, which currently necessitates a manual process (in a local cmake build directory). Maybe there's a way to do this within GH Actions, we shall see.

[gotmax23]

It's definitely possible to have a Github Action to create releases and upload artifacts to them, but authenticating with the rpm.org webservers and signing the tarballs (if RPM still does that) may prove more difficult.

[dmnks]

Based on a separate discussion with @pmatilai, let's go with simplifying the tarball creation, by separating the documentation artifacts (doxygen) into a tarball on its own, and uploading that (via a GitHub action), and optionally the main tarball also, to the given GitHub release so that it shows as an additional "asset". Bundling the documentation was just a convenience thing for packagers (esp. for build environments where doxygen may not be available) but it really isn't necessary as long as we provide the tarball alongside.