Make the `action.yaml` file more readable and improve README

[mayaCostantini]

Is your feature request related to a problem? Please describe.

As the action is now available on GitHub Marketplace:

• The action.yaml file should be made more readable for users to find description and inputs/outputs easily
• The README file should be updated with clearer instructions on what Thoth is, how to set up the action, how to specify parameters, links to doc, etc.

[mayaCostantini]

/sig stack-guidance
/priority important-soon

[mayaCostantini]

Closed by #57