docs(dlp): add DLP Overview page + surface stub/env-var gaps

[cdot65]

Summary

• DLP section in the sidenav was structured differently from peer sections under Runtime Security — every other peer (Guardrail Optimization, Runtime Security itself, Red Teaming, Model Security) ships an Overview page; DLP did not.
• Adds runtime/dlp/overview.md with the 20-command map, auth/env, resource model, shared merge-patch UX, and a "common gotchas" block.
• Surfaces three documentation gaps callers were hitting:
  • profiles delete is a stub that exits 2 with the patch idiom — now in the commands table + dedicated section on profiles.md.
  • PANW_DLP_ENDPOINT was set in .env.example and read by the config loader, but never reached reference/environment-variables.md.
  • DLP was not reachable from the Runtime Security overview grid — added a card.

Files

• docs/runtime/dlp/overview.md (new) — 20-command map, resource model (mermaid), shared patch UX, gotchas
• docs/SUMMARY.md — Overview as first child under DLP
• docs/runtime/dlp/profiles.md — delete row + ## delete section
• docs/reference/environment-variables.md — PANW_DLP_ENDPOINT row
• docs/runtime/overview.md — DLP card in grid
• .changeset/docs-dlp-overview.md (patch)

Test plan

• mkdocs build passes on CI (literate-nav resolves new SUMMARY entry)
• Sidenav renders DLP → Overview as first child, matching Guardrail Optimization pattern
• Mermaid block renders on the Overview page