docs(mkdocs): drop dead nav entry for dev/engineering.md

The file doesn't exist (the live engineering reference is
planning/engineering.md). mkdocs build --strict was emitting a
'file referenced in nav not found' warning. Drop the broken entry;
docs/index.md now links to the GH URL of planning/engineering.md.

site_url left as-is (fictional RTD URL is unrelated project hygiene
to be addressed separately).

Author: lesnik512

mkdocs.yml

@@ -7,7 +7,6 @@ edit_uri: edit/main/docs/
 nav:
   - Quick-Start: index.md
   - Development:
-      - Engineering Notes: dev/engineering.md
       - Contributing: dev/contributing.md
 
 theme: