[skip-ci] [docu] Generate Doxygen docs with Makefile + GitHub Actions 2

[DuesselbergAdrian]

This Pull request:

Continue Jollys work: This draft PR for testing the doxygen generation github action with our self-hosted runners.
Uses our existing Makefile setup. Initial PR #15337

Status

Succeeded with building only docs for core.
The workflow first builds root without tests and then makes the doxygen documentation. It also uploads it to GitHub as an artifact.

Checklist:

• Test with build from scratch and all documentation folders
• Install latest version of doxygen, qhelpgenerator and other required packages in image at https://github.com/root-project/root-ci-images
• Upload result to website/S3
• Nightlies for different releases
• OPTIMIZATION: use build from existing build workflows instead of building again in this workflow. We could for example add a job in root-master.yml that only builds documentation after the run_nightlies job. Github actions supports defining dependencies between jobs: https://docs.github.com/en/actions/using-jobs/using-jobs-in-a-workflow#defining-prerequisite-jobs

This PR fixes #

[DuesselbergAdrian]

@ferdymercury I have some problems with this part: gzip $(DOXYGEN_IMAGE_PATH)/ROOT.qch in the Makefile. Sometimes this file cant be found, even though I enable GENERATE_QHP = YES and install qthelpgenerator in my workflow.
Maybe you can help me with that. Do I need this ghp generation ? Thanks a lot