[Synthetics] Update troubleshooting section

[devcorpio]

Context

Over the last few months, our customers have been asking for help to solve certain cases, from those cases, we have identified certain concepts that should be properly described in the troubleshooting section. This way, our customers will have more certainty and they will know what to do.

Summary

All the topics/concepts pending to include:

• how to enable debug logging in monitors - https://github.com/elastic/sdh-synthetics/issues/45#issuecomment-1932439913
• how to troubleshoot dns - https://github.com/elastic/sdh-synthetics/issues/45#issuecomment-1917508048
• explain the current limitation it's not possible to provide a custom CA ([Heartbeat] Check certificates for Browser based Synthetic monitors (step 1 main URL) beats#22326)
• explain the current limitation synthetic browser might fail in air-gapped attempting to download the CRL or check the certificates against OCSP, and possibly suggest the workaround of faking the CRL endpoint via /etc/hosts and make it fail fast (otherwise it times out) - https://github.com/elastic/sdh-synthetics/issues/45#issuecomment-1938853094
• explain the possible reasons they are seeing common errors like "ERR_HTTP2_PROTOCOL_ERROR" error - https://elastic.slack.com/archives/CE4MRBU48/p1709892248094769?thread_ts=1709874871.118569&cid=CE4MRBU48