`ng_help`: List every available command

[bxyu-nvidia]

Use cases, pain points, and background
Right now users don't have any idea what commands are available in Gym unless they scour the README. Even if you know the command exists, you have to flip through the readme to try to find what the command name is.

Description:
Implement a command ng_help that displays all available commands and quick descriptions of each.

Design:
Can probably use tomllib to parse the pyproject.toml to get the full list under project.scripts.

Out of scope:

Acceptance Criteria:

• Running ng_help displays a list of commands and their respective decisions.
• Add a sanity test
• Add an assert inside the ng_help implementation to check that all the commands in the pyproject.toml are accounted for. Ensure that missing a command fails the sanity test.
• Add ng_help to the top of the README