Docs/health endpoint

[arekborucki]

Proposed changes

Adds a "Monitoring server" subsection under the HTTP transport docs, documenting the optional monitoring HTTP server exposed when running with --transport http:

• documents the /health route (returns {"status":"ok"}) and the /metrics route
• explains that the monitoring server starts only when both --monitoringServerHost and --monitoringServerPort are set
• documents --monitoringServerFeatures (default health-check, optional metrics)
• notes the deprecated --healthCheckHost / --healthCheckPort aliases
• adds a runnable example with a curl health check

Checklist

• I have signed the MongoDB CLA

[nirinchev]

There's already a PR that achieves mostly the same goal: #1267.