Document how to use Briefcase in CI

We should add a topic guide for how to incorporate Briefcase into a CI setup.

This includes:
* On a pull request, running:
- code style checks
- checking for errors in documentation
- running the test suite
- checking for release notes
* Tagging a new app release.
* Signing a released app.
* Publishing a new release.

This will, by necessity, be a very opinionated guide; I would currently expect it to reccomend:
* Github Actions for CI configuration
* towncrier for release notes
* flake8 (possibly mentioning black)
* sphinx for docs
* tox as a common point of testing configuration
* pytest to run test suites

Traveltips currently has an incomplete CI configuration, but can serve as a good "working example". Briefcase has a much more complete CI configuration.

Start by reviewing the incomplete CI configuration in Traveltips and the more complete Briefcase CI configuration mentioned in the issue. Use these as working examples while outlining pull-request checks, release tagging, signing, and publishing. Done means a topic guide covers the requested workflow and explains the recommended tools, including GitHub Actions, towncrier, flake8, Sphinx, tox, and pytest.

Written by the indexing model from the issue text.