[Feature] Add generated manual pages as artifact for download

[wenzeslaus]

I would like to download addons documentation after a build in CI to inspect how were the resulting pages build.

Expected behavior

After a run of the .github/workflows/ci.yml, there should be an artifact to download which is a ZIP file with all the content of the documentation directory ($GRASS_INSTALL/docs/html).

The retention time can be short, just couple days, maybe 3 (the ZIP might be large).

System description

• Operating System: Linux (Ubuntu VM in GitHub CI)

Additional context

• YAML editing and checking of GitHub Actions documentation is needed.
• Examples: Artifact from Sphinx-generated documenation, Artifact from testing.
• g.citation: get year from file attributes and enrich bibtex #677 and .github/workflows: fix MODULE_TOPDIR ENV var path value #806 made me think of this.
• This makes sense for the core (grass) repository too, but given the larger contributor base, it is more important for the grass-addons repository (almost the same CI code will be needed).