Fix all warnings in RTD builds

### 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

Start with craft_cli/helptexts.py, especially the HelpBuilder docstrings named in the Read the Docs warnings, and reproduce the RTD documentation build. Investigate why CommandGroup and BaseCommand resolve to multiple targets, then confirm the build completes without these warnings.

Written by the indexing model from the issue text.