Add a init subcommand

[sebbacon]

This should replace the current "make a repository using a template" workflow.

For now, it can just download the code from the current research template.

The reason this is better is that it allows us to grow new command line options, for example, for updating the GHA tests; it also means we don't have to explain what a Github template is, etc.

The documentation should also be updated. Also here

[evansd]

Further thought: this should obviously check if the project has already been set up and not automatically overwrite existing files. But if it does find existing files it could prompt you to run init --update (or something) which would try to update the project to match the latest version of the template. I'm thinking specifically of the github actions here which we don't expect the user to edit, but there might be other things we can safely update as well.

[sebbacon]

Oh, I wrote up ideas about this a few months ago! opensafely-core/cohort-extractor#327