PR documentation builds

[josh146]

Currently, if the environment variable MOCK_DOCS is set, the setup.py file will skip the building of the wheels. This is required for ReadTheDocs, where C++ extensions cannot be built.

Unfortunately, ReadTheDocs environment variables are not passed to PRs and forks, so we cannot activate the option to build the docs for PRs. An alternative option would be to simply mock out the C++ extensions in the conf.py file, like we do for The Walrus.

[trbromley]

@josh146, do we still need something in setup.py that allows us to install PennyLane-Lightning without having things like Eigen?

[josh146]

@trbromley, with the new device, is this issue now closed?

[trbromley]

@josh146 we still have MOCK_DOCS in setup.py so I believe it would still be a blocker for PR docs builds.

Could we remove MOCK_DOCS? 🤔 I'm not sure, the main difference with the new device is removing the Eigen dependency, but we still do load the C++ extensions within the if-statement. Probably the best option would be to follow what is done in the Walrus.

[trbromley]

I believe you can now share env variables with PR builds: readthedocs/readthedocs.org#7182 (comment). Will have a go!

[mlxd]

Docs now build in each PR. Closing.