docs: Add a load testing guide under Operations

[abdulawalarif]

Summary

• Adds an Operations page for measuring a production-like Serverpod server under concurrent traffic, with Locust as the default path and the generated Dart client for typed, authenticated, and streaming scenarios.
• Makes it explicit that Serverpod does not ship a load-testing tool, and that withServerpod is for correctness tests, not throughput.
• Covers the HTTP POST JSON call shape, what to measure in Insights, health probes, session logs, and Postgres, plus settings that silently invalidate a run (dev mode, JIT, pool of 10, session-log write amplification, single isolate).
• Cross-links the page from Operations overview, testing get-started, logging, and health checks.

Related to serverpod/serverpod#3059. This covers the documentation ask. A first-party load-testing tool is still out of scope.

Test plan

• Open Concepts → Operations → Load testing and confirm the sidebar label and page title.
• Follow the Locust example against a local --mode production server and confirm POST /example/hello with a JSON body is the path shown.
• Click every in-doc link (testing get-started, endpoints, Insights, logging, health checks, configuration, connection pool, streaming, file uploads, authentication, future calls, community Locust scripts).
• From Get started with testing, Logging, and Health checks, confirm the new Related/Next entries land on the load testing page.

[abdulawalarif]

Hi, Jamiu!
Renamed to 07-load-testing.md and moved Load testing last in the overview so it follows Scalability

[abdulawalarif]

Hi Jamiu, addressed the noun-led front matter and health probe wording.