Document (some?) k6 CLI commands

[oleiade]

We have noticed when writing documentation for the k6 cloud run --local-execution --no-archive-upload command that finding a place to document for such behaviors wasn't trivial or obvious.

We think it might be worth having a documentation section dedicated to k6 CLI commands. Maybe we don't need to document all of them, but some of the major ones, at the very least, with a reference for their behavior, why they exist, and when they should be used, could be really useful.

cc @heitortsergent