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.

Co-authored-by: Paul Moore <p.f.moore@gmail.com>

It wasn't clear what "those dependencies" refers to.

Co-authored-by: Paul Moore <p.f.moore@gmail.com>