Add maxdepth option to limit subcommand nesting

[coloursofnoise]

I wanted to be able to use the :nested: full display while keeping the documentation for nested sub-commands on separate pages from the main command.

If this is already possible please let me know.

[stephenfin]

@coloursofnoise Wow, I totally missed this. Sorry. If you're still interested in this, I'm happy with the approach proposed. You'll need to rebase this and add docs, a release note and tests. I'm happy to merge this once done.