Add minimal installation and usage docs for pulp-cli-deb

[quba42]

Since we have zero documentation on how to install and use the pulp-cli-deb plugin for pulp-cli, users do not know it exists, and either miss out, or ask for it, or even try to start implementing it themselves: #595

We should at least add some documentation (perhaps an info box included in several places throughout the workflow docs), perhaps also a paragraph in the feature overview docs. That tells users that pulp-cli-deb exists as a plugin for pulp-cli, how to install it, and how to run pulp deb --help.