Populate Fundamentals/Protocols doc for ping

[salmad3]

Done Criteria

• What is Ping
  • Request/response in p2p
  • Dialling/listening
• Ping in libp2p
  • Abstraction of the official spec

Why Important

• A better way to explicitly introduce the ping protocol.
• Provides a better reference point for other content.

Notes

• Can use content distributed across the docs.
• Good term to add to the glossary.
• Follows Protocol Basics section #249