Add information about optional/required fields in Actions documentation

[simar7]

The documentation currently lists fields that our Actions accept but doesn't always list which field is Optional vs. Required. This information however is available within the comments for each Action in the sample cfg.yaml file we used to have: https://github.com/aquasecurity/postee/blob/main/cfg.yaml

Documentation exists here: https://github.com/aquasecurity/postee/blob/main/docs/actions.md

Acceptance Criteria:

• Clearly document each field for every Action to be Required or Optional as a separate column within the same table of all fields.