Adds docs/testing.md as the authoritative testing map (consolidated
from docs/contributing.md §8, now shrunk to a pointer), and a
generated docs/endpoint-index.md via scripts/generate-endpoint-index.py
(hooked into scripts/update-openapi.sh). Updates docs/README.md's
reading order and removes the "still to come" placeholder.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
