Codecov Slack Integration Notification Configs for Checks Failed

[sdzhong]

What product do you want to improve?
Codecov Slack Integration https://docs.codecov.com/docs/slack-integration

Is your feature request related to a problem? Please describe.
I want to be notified when something fails. At the moment, every time Codecov completes its analysis and posts a comment on the PR, it also sends a Slack message.

Describe the solution you'd like
I would like a way to configure the Codecov Slack Integration to make it less noisy and only alert when checks have failed.

Describe alternatives you've considered
Currently, the Codecov Slack Integration is all or none for slack notifications.

Additional context
https://docs.codecov.com/docs/slack-integration is not clear on how to enable notifications on that page. Having those details highlighted (and not hidden in a table) would be more readable.