Add slack notifications to release automation PR

[tannalynn]

The github action to create the release notes PR on the docs site can fails without us noticing pretty easily. We should add slack notifications on failure so that we can keep track of any issues that come up.
Also, we could do this for any other automation PR that might be helpful for us to keep track of.

[workato-integration]

https://new-relic.atlassian.net/browse/NR-221440