binary-extensions: add cibuildwheel and multibuild

[henryiii]

This adds a mention of pypa's cibuildwheel and multibuild here before going into the (highly outdated, currently) specifics of building wheels. For most users, this likely is the correct place to go, rather than trying to set things up themselves. For those who might want to do things themselves, the following sections still need updating.

[bhrutledge]

Thanks for starting to improve this doc. I have no direct experience with binary extensions, so it's a good read for me. There's one spelling fix, plus some suggestions for clarity.

[henryiii]

Thanks, @bhrutledge!

[henryiii]

There's no multibuild project summary, so this PR is failing after the change. I think the best fix would be to add a multibuild section under non PyPA project, rather than to revert to a link. Will make a PR for that that this PR will then depend on.

[pradyunsg]

LGTM, although this whole guide likely needs to be updated and completed. :)

[henryiii]

I'm working on it, in small pieces. I'll tag a few possibly interested parties when I make the next PRs (likely after PyHEP and SciPy, so in a couple of weeks at the earliest)