[9.18] new: doc: Document opt-in 🤖 marker for agent-authored issues and MRs

Add short notes in CONTRIBUTING.md telling automated agents to append
🤖 to the title of issues and merge requests so they can be routed
through the streamlined agent triage/merge process.

Backport of MR !11861

Merge branch 'backport-ondrej/agent-contributing-9.18' into 'bind-9.18'

See merge request isc-projects/bind9!11863