.github/workflows: Integrate PR Preview workflow

[AnnsAnns]

Contribution description

I added the guide preview CI workflow we tested a few months ago. To enable it on a PR it needs to have the "CI: preview docs" label to prevent bad actors misusing this.

Since the PR Previews use a folder on the gh-pages branch, I also needed to change the the CI deployment action to a different one, since it didn't have support for excluding certain folders from the cleanup.

The doc related CI workflows were also the only ones that used 4 spaces, I fixed that formatting error.

Testing procedure

I uh, don't exactly know how best to test this working in our repository. It worked for my own blog but the environment is fairly different. Worst case this just fails, best case it works. Either way it shouldn't impact anything even if it breaks, (esp. since its guarded by the aforementioned label).

Issues/PRs references

[riot-ci]

Murdock results

✔️ PASSED

51cddf9 fix: use full path

Success	Failures	Total	Runtime
1	0	1	01m:15s

Artifacts

• Documentation preview

[AnnsAnns]

From searching for the error, it appears like there is a way to approve certain workflow runs on a per-PR basis which needs to be done here (the alternative would be to send secrets to fork CI workflows which probably isnt something that should be done on RIOT). Looking into it.

[AnnsAnns]

Yeah no, I'll report on the findings in tomorrows meeting but the TLDR is that this is not well translatable to the RIOT PR workflow without breaking some (very intentional) security checks Github has put into place. I tried integrating this into Murdock but Murdock is way too gigantic, complex and undocumented hard to understand as an outsider.