Add a dedicated "Build system interface" reference section

[pradyunsg]

This reworks a substantial portion of our existing build system related
documentation, putting it all in a single location.

There are certain areas that have been rephrased or rewritten:

• The setup.py interface is now considered an implementation detail.
• The setup.py interface is explicitly noted as a legacy interface, due
  for removal.
• The pyproject.toml interface now has clearer documentation on how it
  is used and how it operates.
• Clearer separation of --use-pep-517 and --no-build-isolation, to
  make the difference between the two clearer.

[pradyunsg]

Toward #9475.

[pradyunsg]

@pypa/pip-committers I'd really like for someone to take a proper look at this PR -- it's a decently large change, and very much worthwhile IMO. However, it is also one of the places where we should get the nuances and phrasing right and to be something that we're comfortable with.

Here's the link to the preview: https://pip--10497.org.readthedocs.build/en/10497/ -- you'll wanna look under Reference. :)

[pfmoore]

Overall, this looks good. I've added a bunch of comments, but they are mainly just clarifications of things I think are a bit unclear, and one or two nitpicks.

The most substantial query is around the setup_requires explanation, which I think might have some inaccuracies.

[pradyunsg]

@pfmoore That should be all the feedback addressed! I'd like you to take a look at the easy_install/setup_requires thing again, since that's the only bit where I've had done significant stuff.

PS: I added you as a co-author on one of the commits, since a lot of the stuff in it is based on copy-pasting what you've suggested. :)

[pfmoore]

LGTM. One spelling nit is all.