feat: add audit events to datadog guide

[markphelps]

• Adding a guide on how to send audit events to datadog
• Fixing a couple docs issues

[erka]

@markphelps Having such a guide would be great.

I have a follow-up question about the body data since I'm unable to verify it myself. In the Datadog API documentation, all the example logs are sent as arrays. Additionally, there are a few reserved attributes that could potentially aid in the aggregation console. Here’s an example from their site:

[
  {
    "ddsource": "nginx",
    "ddtags": "env:staging,version:5.1",
    "hostname": "i-012345678",
    "message": "2019-11-19T14:37:58,995 INFO [process.name][20081] Hello World",
    "service": "payment"
  }
]

Probably we could have ddsource set to flipt in the guide.

[kodiakhq]

This PR currently has a merge conflict. Please resolve this and then re-add the automerge label.