Failed events alerts DQD

[patofp]

Added basic guide for new failed events.

Not sure if we should remove or mark as deprecated the old failed events alert.

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[gibbok-snowplow]

LGTM
@patofp just few minor suggestions:
Maybe add a section for Slack in the Troubleshooting, perhaps link to any useful documentation they may have

[mscwilson]

Seems a little overkill to mark the whole existing Failed events section as Legacy? Is the data quality dashboard still a thing?
I think it's confusing to have two sections named something about failed events within the Data quality section, can you combine them please

[jborlase-snowplow]

My two cents - could we rename "Legacy Failed events" to be "Failed Events" and then nest the new pages under an "Alerting" section within "Failed Events"?

[gleb-lobov]

@jborlase-snowplow does this look better?

[jborlase-snowplow]

LGTM!