Test for building the docs

[jarlsondre]

Currently, the docs are often forgotten (at least by myself) when working on features, making our readthedocs page contain errors etc. Therefore, it would be nice to check that the docs build properly whenever we create a pull request. Right now, when we do make http in docs/, it let's us build despite a bunch of warning. However, by adding -W´to SPHINXOPTS, the build will fail if it encounters any errors. Then it should be possible to just add cd docs/ && make clean && make http` to the GitHub Actions to check that the build works.