Reduce size of sphinx-gh output (#6601)

Reduce size of sphinx-gh output (#6601)

This PR reduces the disk size of the docs generated by make sphinx-gh.
Apart from reformatting the cmake source, we now not only remove the _sources folder, but also .doctrees (essentially the sphinx cache) and _static/fonts/ (the fonts that are actually used live in _static/css/fonts).
In combination, this now reduces the disk size from about 20MB (sphinx) to less than 6MB (sphinx-gh).
Furthermore this PR only uploads the generated documentation if it differs from whatever we currently have for master.
This is relevant to make the docs-ci repository smaller (which already has more than 5GB...)