Play Framework

docs: drop stale samples-and-tests/facebook-oauth2 link in libs.textile

PF-84 follow-up — fix the one stale reference that fell outside the
three agents' ownership scope. Replaces the broken pointer with a brief
inline outline of the OAuth2 flow using the WS client.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>