[Documentation] Stub Out or Prune References to Advanced/Deployment Docs

[jmprieur]

Several documentation pages (e.g., quickstarts, README) refer to advanced or deployment topics (e.g., advanced/incremental-consent-ca.md, deployment/azure-app-service.md, advanced/api-gateways.md, and others). These files are missing, creating a gap between promises in the navigation structure and the actual content.

Required Fixes:

• Create initial stub pages for the referenced advanced and deployment docs (with placeholder structure and clear TODOs), or prune links if these docs are intentionally out of scope
• Review cross-references for mislinks and maintain consistency throughout navigation

PRs must target: feature/doc-modernization branch.

Affected structure: docs/advanced/, docs/deployment/, and all navigation/crosslinking referring to these topics