Doc: CI workflows

[LecrisUT]

The documentation could use some improvement to point to:

• How to cache the containers, venv, etc.
• How to re-use the artifacts, e.g. in order to combine the tasks and upload to PyPI
• How to interact with multiple CI environments. E.g. CircleCI supports dispatching workflows from github action, and with a bit of curl, a simple workflow can be written to wait for it, download and combine the artifacts

[joerick]

1. A caching section in the FAQ would be great, if you can contribute it.

2. We do already show an example of this in https://github.com/pypa/cibuildwheel/blob/main/examples/github-deploy.yml . I wouldn't want to go much further than this though, I don't want to duplicate docs from Github Actions itself.

3. This might get into the realm of too much detail for cibuildwheel - we're responsible for building the artifacts, if you want to do something fancy with them after that, I think you'll have to do your own research.