Support for Sphinx parallel build

[bluetech]

I was trying out sphinx-build -j auto in the pytest repo to maybe speed up the docs build a bit. It fails with the following error:

NotImplementedError: merge_domaindata must be implemented in <class 'pallets_sphinx_themes.themes.click.domain.ClickDomain'> to be able to do parallel builds!

Link to merge_domaindata.

I haven't been able to complete a -j build so I'm not sure if it's actually worth the effort.

[davidism]

It's unclear from that link how to implement it. I'd be happy to consider a PR, but won't have time to work on this myself any time soon.

[bluetech]

Yes it's not very clear, but after looking into it, it's meant for Domains which use self.data, which ClickDomain and JinjaDomain don't (either directly or indirectly). So the implementation can just be a no-op. I will submit a PR.

BTW, after testing in pytest, -j4 brings the docs build time from 29s to 23s, so doesn't help that much, but might as well support it.