⚙️ Config: Sample appsettings for rate limiting

[jongalloway]

Provide a sample rate limiting config for the demo.

Context

• RateLimitingOptions exists but lacks sample configuration in the demo.

Scope

• Update samples/Demo/appsettings.json to include a commented sample RateLimiting section (Enabled, RequestsPerWindow, WindowSizeInMinutes, EnableIPBasedLimiting, EnableClientBasedLimiting, ClientIdHeader)
• Add a short paragraph in doc/configuration-format-updates.md explaining the keys

Acceptance Criteria

• Demo contains a ready-to-tweak rate limiting sample

Labels: configuration, docs, good first issue