epic: Echo Server/Push API docs

[HarryET]

This contains everything we need to document for Echo Server and the Push API

• push_url TTL, https://github.com/WalletConnect/rs-relay/pull/493#discussion_r983682191
• The spec
• Security (mTLS)
• Hosted platform

[HarryET]

@finessevanes, am I okay to assign you to this?

[HarryET]

@finessevanes do we still need this issue or has the stuff here already been included?