Add User Documentation

[n-boshnakov]

What would you like to be added:

I would like to suggest that user documentation be added for the ACL extension.

As a general idea, the user documentation could contain:
• How to enable the extension
• How to configure the extension
• Tutorials (if needed)

Good examples of user documentation:
• https://github.com/gardener/gardener-extension-registry-cache/blob/main/docs/usage/registry-cache/configuration.md
• https://github.com/gardener/gardener-extension-provider-aws/blob/master/docs/usage/usage.md

The contents of the main README.md could also be split and used as a starting point for the documentation.

Why is this needed:

This would help users of the ACL extension get started with it more easily and learn how to complete various most common use cases.