Add a new docs section "Building a package"

[rgommers]

From #634 (comment):

@dnicolodi suggested:
I wonder whether it is time to have a Building a package top sections in the documentation with the details about build isolation, venv and some hints about cibuildwheel and co for building redistributable wheels.

@rgommers replied:

Yes, I agree. This is the cause of a lot of the questions/issues we are receiving. I also just searched for auditwheel and it's not in our docs (except in the non-public explanations/design-old.rst).

I won't be able to work on that in the next few days though, since it'll take a few hours at least to do it right. Should we open a new issue for that first, and scope it? I think I'd prefer to address your one comment here, and then have a new PR for the new docs page, containing:

• The content added here
• Content from design-old.rst
• Mentioning all the relevant tools.
• I'd like to refer to https://pypackaging-native.github.io/meta-topics/build_steps_conceptual/#building-and-installing-or-uploading-artifacts, which explains a lot of things that are relevant here.

We do indeed get lots of variations of the same questions that are conceptual issues with how Python packaging works and how meson-python fits in. I'm under no illusion that all users are going to understand build isolation and the difference between local and redistributable wheels, but it'd be nice if we could reply with a one-liner linking to the docs.