Network Design - Renamed Gossip to Peer Sharing

[bolt12]

Description

This PR updates the existing Network Design document to use the right words when it refers to Gossip. In essence renames (when appropriate) "gossip" to "peer sharing".

Checklist

• Branch
  • Commit sequence broadly makes sense
  • Commits have useful messages
  • New tests are added if needed and existing tests are updated
  • If this branch changes Consensus and has any consequences for downstream repositories or end users, said changes must be documented in interface-CHANGELOG.md
  • If this branch changes Network and has any consequences for downstream repositories or end users, said changes must be documented in interface-CHANGELOG.md
  • If serialization changes, user-facing consequences (e.g. replay from genesis) are confirmed to be intentional.
• Pull Request
  • Self-reviewed the diff
  • Useful pull request description at least containing the following information:
    • What does this PR change?
    • Why these changes were needed?
    • How does this affect downstream repositories and/or end-users?
    • Which ticket does this PR close (if any)? If it does, is it linked?
  • Reviewer requested

[bolt12]

closing in favor of #4019