cli: add generate-docs command to generate skeleton pack readme

When authoring packs, we want to encourage writing useful documentation whilst making every effort to streamline this process. Luckily, Nomad Pack has both a way to read the root variables file, and render arbitrary templates. We could therefore use these internal processes to render a basic README based on the passed root variables and pack information.

Start by locating the CLI command registration and the existing code that reads the root variables file and renders arbitrary templates. Trace how pack information is represented and passed through those paths. Done means a generate-docs command can use the passed root variables and pack information to produce a basic skeleton README.

Written by the indexing model from the issue text.