Dedicated `CLI` section in the docs

Since the CLI is probably the most straightforward way of using `atldld` I think we should have a dedicated section in the docs for it.

Locate the documentation entry point and any current CLI-related material; the issue does not name a file or test. Define the dedicated CLI section's scope from the existing documentation, and consider the work complete when the CLI has a clear, navigable section in the docs.

Written by the indexing model from the issue text.