feat(develop): Add monitor category to rate limiting docs

Author: lcian

develop-docs/sdk/expected-features/rate-limiting.mdx

@@ -87,6 +87,7 @@ While these [data categories](https://github.com/getsentry/relay/blob/master/rel
   - `default`: Events with an event_type not listed explicitly below.
   - `error`: Error events.
   - `transaction`: Transaction type events.
+  - `monitor`: Monitor check-ins.
   - `span`: Span type events.
   - `log_item`: Log events.
   - `security`: Events with event_type `csp`