Document how to recover public keys from ECDSA signatures

[netrome]

Background

From https://nearone.slack.com/archives/C07UW93JVQ8/p1762181188765319?thread_ts=1762180518.613449&cid=C07UW93JVQ8

Currently we provide a recovery ID with our ECDSA signatures. We should provide documentation so that people know how to use this recovery ID to recover public keys from the signatures.

User Story

As a chain signatures user I'd like to know how to use the returned recovery ID.

Acceptance Criteria

We have documentation explaining how the returned recovery ID works.

Resources & Additional Notes

No response