docs: add embedded PDP documentation

[beonde]

Summary

Documents the new embedded PDP feature added in capiscio-server#36.

Changes

docs/how-to/security/policy-enforcement.md

• Restructured guide into Option A: Embedded PDP and Option B: External PDP
• Added embedded PDP setup steps (enable, review decisions, tighten enforcement)
• Documented bundle staleness behavior per enforcement mode
• Documented bundle endpoint (GET /v1/bundles/{workspace_id})
• Updated configuration reference with new env vars:
  • CAPISCIO_EMBEDDED_PDP
  • CAPISCIO_BUNDLE_POLL_INTERVAL
  • CAPISCIO_BUNDLE_STALENESS_THRESHOLD
• Updated CAPISCIO_WORKSPACE description to mention UUID requirement

docs/reference/server/index.md

• Added "Embedded PDP" to the server feature list

Related

• capiscio-server#36 (implementation)