[RF DOCS] Review Action Mailbox guide [ci-skip] (rails#50973)

Review and improve Action Mailbox guide, with some sections added/improved:

- Installation steps
- Processing Incoming Email - more of a step by step of the flow
- Mail object and it's fields
- List of Inbound Email 'status' values
- Routing and Incineration

Co-authored-by: Carlos Antonio da Silva <[email protected]>

Author: bhumi1102