Infra: Use consistent 'make html' and 'make dirhtml'

[hugovk]

I keep forgetting what make render and make pages actually do, and to be sure need to check how the Makefile calls build.py and how that calls sphinx-build.

Let's add make html and make dirhtml aliases.

These are the actual Sphinx builder names, and the same Makefile targets are used by other Sphinx/Python projects, including CPython docs and the devguide, so we can use muscle memory.

(They're also the default for new Sphinx projects.)

And then I don't think we need the old make render and make pages and can mark them as deprecated?

[CAM-Gerlach]

LGTM, makes sense (I'd get confused too).

Though to be honest I don't ever actually use the Makefile (or even build.py, since it basically just wraps a limited subset of Sphinx's interface)—I just run python -m sphinx . build instead, which does exactly the same thing but gives me the full power of Sphinx's native CLI, including writing out only one specific PEP, writing to separate build dirs, passing -b -X dev to get Python-level warnings, etc. And, of course, it lets me use my Conda env and test different Sphinx and tool versions vs. the hardcoded venv.

[Rosuav]

LGTM. I've only ever used the defaults but consistency is always a good thing.