Add rule catalog

[matthieucan]

Add the list CLI command, which is able to:

• display in the terminal the list of known rules, along with their description
• generate a markdown output of the know rules, suitable for website documentation

To that end:

• Add necessary mkdocs configuration.
• Allow Config to have its values overloaded. CLI values overwrite pyproject.toml values.
• Fix project entry point, to go through __main__.py (which sets up logging).
• Add -n/--namespace option to CLI commands, to overwrite the rule namespaces.
• Refactor RuleRegistry, to instantiate rules automatically, and only look for the namespace list in the provided config.