Complete upstream docs review of the OIDC code flow authentication upstream guide security-oidc-code-flow-authentication.adoc for Jan 15th.

This task includes the following scope.

1. Check the upstream source on main.
   • Run Vale and other helpful content quality checkers. 
   • Fix red errors and orange serious warnings only or legal-risk issues
   • Complete style and language checks. Ensure content is polished and ready for RHBQ publication (process for downstream publication tbc).
   • The review does not include restructuring. If a guide needs restructuring, raise a GitHub issue.
2. Send for peer review by another writer - Peer reviewer to focus on critical issues only like typos or serious quality issues.
3. Send for SME review to Sergey. Sergey to merge once review complete and approvals received.