Update the README with more relevant information

What is your Merge about?

If this is your first merge, please use the Initialization Template.

This merge is updating the README of this repo to point to more relevant information and include usage info for the zarf-agent image.

For more information please visit Iron Bank Documentation

Note: Please be sure you an issue related to this merge request and the issue is labeled ~"Hardening::Review", this will initiate a CHT member review your merge request.