[Docs]: Add Documentation for monitoring

[amanagarwal042]

Hi Team, I am one of the maintainers of OpenLIT and we support Observability for Agents built using ControlFlow, It is all OpenTelemetry-native so the traces and metrics can be sent to any platform like Grafana or any OSS OTel tools

Figured the mention here might be useful for developers using ControlFlow hence adding the documentation page. Lemme know how I can help you folks with this further.

[amanagarwal042]

Hey tagging @jlowin as not sure who to tag for review :)