Fix a couple of documentation issues

[ZedThree]

• Typo in the CMake messages for building the docs
• Latest version of breathe added a new option for the auto-doc tool that we call in a slightly weird way

[ZedThree]

Using my admin privileges to merge this so that the readthedocs checks pass, otherwise other PRs have check failures that aren't relevant. Shout at me later if this turns out to cause problems 🙂

[dschwoerer]

I wanted to ask whether the targets should be renamed from sphinx- to manual- to match the autotools version and make it easier to find them? Or would make doc(s)- be even easier to guess?

More general, do we need to discover sphinx at configure time? I think it is nice to only check for sphinx as the user tries to build the docs?

[ZedThree]

I wanted to ask whether the targets should be renamed from sphinx- to manual- to match the autotools version and make it easier to find them? Or would make doc(s)- be even easier to guess?

Yeah that's a fair-point. I was translating manual/Makefile which uses the sphinx- names, but the top-level Makefile calls them manual-. So it would be better to use that instead. docs is also a good name.

More general, do we need to discover sphinx at configure time? I think it is nice to only check for sphinx as the user tries to build the docs?

Currently, CMake tries to find them if you configure with BOUT_BUILD_DOCS, which is default off, so turning it indicates the user probably wants to build the docs. More practically, it's just much easier with CMake to find things at configure time.

[ZedThree]

Now that I've merged this, RTD is pulling in Sphinx 4.0 which removes app.add_stylesheet, so there's a fix for that incoming shortly. I'll also add a target docs that builds sphinx-html