Fix all warnings in RTD builds

[bepri]

What needs to get done

RTD builds have warnings that are currently being ignored:

/home/docs/checkouts/readthedocs.org/user_builds/canonical-craft-cli/checkouts/340/craft_cli/helptexts.py:docstring of craft_cli.helptexts.HelpBuilder:1: WARNING: more than one target found for cross-reference 'CommandGroup': craft_cli.CommandGroup, craft_cli.dispatcher.CommandGroup
/home/docs/checkouts/readthedocs.org/user_builds/canonical-craft-cli/checkouts/340/craft_cli/helptexts.py:docstring of craft_cli.helptexts.HelpBuilder.get_command_help:1: WARNING: more than one target found for cross-reference 'BaseCommand': craft_cli.BaseCommand, craft_cli.dispatcher.BaseCommand

Why it needs to get done

For better maintainability and predictability of docs builds