Add AI coding assistants guidance to CONTRIBUTING.md

Add AI coding assistants guidance to CONTRIBUTING.md

Adapted from the Linux kernel's Documentation/process/coding-assistants.rst
to the BIND 9 context. Adds three subsections under the existing
"Guidelines for Tool-Generated Content" section:

- Licensing and legal requirements (MPL-2.0, SPDX identifiers).
- Signed-off-by and Developer Certificate of Origin: AI agents must
  not add Signed-off-by trailers; only the human submitter may
  certify the DCO.
- Attribution: the Assisted-by: AGENT_NAME:MODEL_VERSION trailer
  for recording AI involvement, with an explicit prohibition on
  AI-added Co-Authored-By trailers (Co-Authored-By designates a
  human co-author who shares responsibility).