Document bi-directional data transfers

# Feature Request

The Bi-Directional data transfers is a new feature and it definitely deserves some docs on this website:
https://github.com/eclipse-edc/Connector/pull/4579

## Which Areas Would Be Affected?

_e.g., DPF, CI, build, transfer, etc._

## Why Is the Feature Desired?

_Are there any requirements?_

## Solution Proposal

_If possible, provide a (brief!) solution proposal._
