Ensure --help examples in docs are always up to date

[simonw]

Ideally I would automatically generate the --help output shown in our docs, but I don't think I can get that working with readthedocs.

Instead, I'm going to add a unit test that checks that those extracts in the documentation match the current output of the --help command.

[simonw]

Easiest way to do this I think would be to make those help blocks separate files in the docs/ directory (publish-help.txt perhaps) and then include them with a sphinx directive: https://reinout.vanrees.org/weblog/2010/12/08/include-external-in-sphinx.html

[simonw]

I decided against the unit tests, instead I have a new script called ./update-docs-help.sh which I can run any time I want to refresh the included documentation: aec3ae5#diff-cb15a1e5a244bb82ad4afce67f252543

[simonw]

Actually I do like the idea of a unit test that reminds me if I've forgotten to update the included files.