docs: documentation for slack integration

[syphernl]

The Atlantis docs only mentioned how to configure the Slack token, but not how to actually use it.
This is what I have cobbled together based on what I was able to find in the issues of this repository.

[posquit0]

Where should I put the configuration for slack webhook?

[kangman]

@syphernl similar to @posquit0 thank you for your documentation, i've tried to set the webhook yaml block in the server config as well as the atlantis.yaml config on my root of my repo but both run into issues with errors like
field webhooks not found in type raw.RepoCfg"

[guilatrova]

@kangman I'm having the same issue. Did you find a fix?

[jamengual]

this :

webhooks:
- event: apply
  workspace-regex: .*
  kind: slack
  channel: my-channel

is a atlantis startup config, you need to pass to atlantis on start up , this is not a atlantis.yaml level config.