docs: drop empty Contributing page

The Contributing page was a one-paragraph stub saying "populated as
concrete contributor-facing material accrues" — a dead click for any
reader who landed on it. Until there's a real contributor pipeline,
having no page is more honest than having an empty one.

Deletes docs/contributing/ and its nav entry in mkdocs.yml. AGENTS.md
remains the agent-facing orientation; if a real contributor doc is
wanted later, it can be lifted from AGENTS.md selectively.

Author: chris-colinsky

docs/contributing/index.md

@@ -105,7 +105,6 @@ nav:
     - openarmature.llm: reference/llm.md
     - openarmature.checkpoint: reference/checkpoint.md
     - openarmature.observability: reference/observability.md
-  - Contributing: contributing/index.md
 
 extra:
   # Hide the "Made with Material for MkDocs" footer.