Plugging the `git` version into the `PROJECT_NUMBER`

[jhlegarreta]

I am wondering whether the action can be instructed to use a custom script that simply calls doxygen with the PROJECT_NUMBER being exported using the git version or a tag name.

If not, can this be done otherwise?

Otherwise, and although the action implies that the documentation matches the HEAD revision, we do not have any indication of the version that we are looking at.

Thanks for maintaining this tool.