docs: adding docling-mcp to the documentation

[PeterStaar-IBM]

📝 Summary

This PR adds a guide for connecting Docling MCP to ContextForge.

The guide documents:

• Local and remote Docling conversion modes.
• Streamable HTTP registration through /mcp.
• Native and Docker Compose deployment paths.
• Gateway discovery and integration verification.
• Filesystem, network, RBAC, and tenant-isolation considerations.
• Troubleshooting for connectivity, SSRF protection, HTTP 421 responses, missing tools, and timeouts.

Docling MCP remains an external service with an isolated dependency environment. This design avoids vendoring upstream code or adding conflicting dependencies to ContextForge.

📏 Reviewability

• This PR has one clear purpose
• The linked issue is not labeled triage
• Unrelated bugs or improvements are tracked in separate issues/PRs
• Tests are not required because this PR changes documentation only
• If AI-assisted, I understand and can explain the generated changes

🏷️ Type of Change

• Bug fix
• Feature / Enhancement
• Documentation
• Refactor
• Chore (deps, CI, tooling)
• Other

🧪 Verification

   Check                         Command                                                                           Status
  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━  ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━  ━━━━━━━━━━━━━━━━━━━━━━━━━━━
   Changed-page Markdown lint    npx --no-install markdownlint-cli2 docs/docs/using/servers/external/docling.md    Passed
  ────────────────────────────  ────────────────────────────────────────────────────────────────────────────────  ───────────────────────────
   Whitespace validation         git diff --cached --check                                                         Passed
  ────────────────────────────  ────────────────────────────────────────────────────────────────────────────────  ───────────────────────────
   Documentation build           cd docs && make build                                                             Pending
  ────────────────────────────  ────────────────────────────────────────────────────────────────────────────────  ───────────────────────────
   Unit tests                    Not run                                                                           Documentation-only change
  ────────────────────────────  ────────────────────────────────────────────────────────────────────────────────  ───────────────────────────
   Coverage                      Not run                                                                           Documentation-only change

  The root make markdownlint command reports pre-existing findings in mcpgateway/alembic/README.md and mcpgateway/toolops/README.md. These files are outside this PR.

✅ Checklist

• Documentation passes targeted Markdown lint
• Tests are not required because runtime behavior does not change
• Documentation navigation includes the new page
• No secrets or credentials are committed
• All example credentials use placeholders

———

📓 Notes

This PR intentionally excludes vendored Docling MCP source, ContextForge dependencies, and Docker Compose services.

Docling MCP runs as a separate Streamable HTTP service. ContextForge registers and federates its /mcp endpoint.

A future PR can add an optional Compose profile after maintainers approve the deployment and maintenance model.

Only check the linked-issue and AI-assisted boxes when both statements are true.