[Utility][Documentation] Add a README section on how to use the portal as a servicer

[adshmh]

Objective

Add a section to the documentation (in utility and/or localnet docs) to describe how to create a Pocket Portal account and use it as a servicer for sending relays as part of development/testing.

Origin Document

Remove the need for running an Ethereum (or any other relay chain) node when sending relays and verifying the results as part of development/testing work.

Config section from #869 shows the need for this:

Goals

• Streamline development/testing process by following a a simple set of instructions.

Deliverable

• Updated documentation and/or a short video

Non-goals / Non-deliverables

• Instructions/support on running an Ethereum (or any other relay chain) node.

General issue deliverables

• Update the appropriate CHANGELOG(s)
• Update any relevant local/global README(s)
• Update relevant source code tree explanations
• Add or update any relevant or supporting mermaid diagrams

Creator: [@adshmh]
Co-Owners: [@Olshansk]