docs: document rate limiting configuration, endpoints, and error format (#515)

[Francis6-git]

Description

Resolves #515. This PR adds comprehensive documentation regarding the server's rate-limiting behavior, default settings, protected endpoints, and error response shapes.

Unlike a per-route implementation, the codebase relies on a single global express-rate-limit middleware mounted at the app root, which has been explicitly documented to guide both contributors and operators.

Scope of Changes

• Documented the global limiter middleware logic (src/middlewares/rate.middleware.ts) and proxy configuration.
• Listed the default protected endpoints under /api/v1/... (such as alert and webhook registrations).
• Outlined the difference in request thresholds based on the MODE environment variable (production vs. other environments).
• Provided the exact JSON schema, HTTP status code (429), and response headers returned during a rate-limit event.

Acceptance Criteria Verification

• Protected endpoints are listed with their default limits
• Time window is stated (15 minutes)
• Environment config override (MODE) is explained
• Rate limit error response shape is documented with accurate fields (type, message, retryAfterSeconds, timestamp)

Documentation Created

• docs/rate-limiting-configuration.md

Closes #515

[drips-wave]

@Francis6-git Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits