Very large gh-pages branch

[soxofaan]

This repo builds the docs for every pull request, and commits that to the gh-pages branch.

that gh-pages branch is now getting very large:

• with currently 75 different PR documentation builds (e.g. see https://github.com/eu-cdse/documentation/tree/gh-pages/pr-preview)
• 11 GB in total (approx 150MB per PR)

I think that is the reason that the github pages based hosting (at https://eu-cdse.github.io/documentation/) isn't working properly

[soxofaan]

I think merged/closed PRs should be removed from the gh-pages branch

[soxofaan]

I manually removed a bunch of closed/merged PR related builds bringing down the stats to:

• 7 doc builds
• 940MB in total

[soxofaan]

again, removed 16 subfolders of old/merged branches, as gh-pages didn't seem to be working at the moment