Infra: override RTD build commands

[hugovk]

ReadTheDocs now has a beta feature to override the build commands:

• https://twitter.com/readthedocs/status/1549145303801946112
• https://docs.readthedocs.io/en/latest/build-customization.html#override-the-build-process

Right now, a whole bunch of dependencies are installed that we don't use.

This can be slow especially as some old dependencies are built from source because they don't support modern Python and don't have wheels available (e.g. readthedocs/readthedocs.org#9118).

Instead, we can just make pages and only install the stuff we need.

This speeds up the build "command" time ~2.5 mins to ~2 mins (uploading following build takes ~100s in both cases).

---

Before

Each build step took:

1. Command time: 2s
2. Command time: 0s
3. Command time: 0s
4. Command time: 0s
5. Command time: 1s
6. Command time: 9s
7. Command time: 33s
8. Command time: 4s
9. Command time: 0s
10. Command time: 98s

Total command time: 147s

Build took 241 seconds (includes upload time)

https://readthedocs.org/projects/pep-previews/builds/17496237/

After

1. Command time: 2s
2. Command time: 0s
3. Command time: 0s
4. Command time: 0s
5. Command time: 0s
6. Command time: 115s

Total command time: 117s

Build took 213 seconds (includes upload time)

https://readthedocs.org/projects/pep-previews/builds/17510658/

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This also adds an option to build.py so we can output the build files directly in the _readthedocs/html required for RTD.

Also fix render.yml: call like make pages JOBS=$(nproc) instead of make pages -j$(nproc).

[CAM-Gerlach]

Kinda out of scope for this PR, but I'm not entirely sure why we're going to all the trouble of manually duplicating the arguments from Sphinx's own build command in the first place, for what appears to be the tiny convenience of typing python build.py instead of python -m sphinx . build for the small subset of users both building locally and not using the Makefile (assuming they remember the bespoke pattern and args to begin with).

The only other thing this file does AFAIK is copy PEP 0 to the root, but the Sphinx extension just does that anyway if not run from build.py by calling the very same function from this file, so it would seem that function should simply be moved into the extension (which would make the latter fully self-contained).

[CAM-Gerlach]

LGTM at least, thanks @hugovk