Documentation specific to the Github Action

[ZIJ]

• Env vars that can be used
• Other options?

Probably makes sense to do smth similar to what we did for AWS-specific / GCP-specific / Azure-specific sections
Either sub-pages or one page for Github Actions for now that later will be split into sub-pages if need be
This is not the same as Getting Started
And not the same as Configuration (that is primarily concerned with digger.yml)

[norman-zon]

Also I would expect documentation specific to the Action on the GHA Marketplace, not just the general README