Create a procedure for using API to manage Slack notifications in dbt Cloud

## Summary
Write a full workflow on managing notifications from dbt Cloud to a Slack channel

Here's how we recommend setting up Slack notifications in the [dbt Cloud UI](https://docs.getdbt.com/docs/deploy/job-notifications#slack-notifications)

## Problem
Using the dbt Cloud Slack channel picker when there are too many Slack channels in a community is difficult to do and can cause problems.

## Solution
Create a procedure for using API to manage Slack notifications for dbt Cloud. This procedure should instruct how to manage notifications from dbt Cloud to a Slack channel, and include an example formatted API request.

The new content should include:
* An API section for the Slack notifications that points to the endpoint you'd use
[https://docs.getdbt.com/dbt-cloud/api-v2#/operations/Create%20Notification](https://docs.getdbt.com/dbt-cloud/api-v2#/operations/Create%20Notification)
* How to find Slack channel ID (at the bottom of the About page for a Slack channel)
* Use an example Curl for doing this?
* Where does this content belong?

### References
[Slack Message](https://dbt-labs.slack.com/archives/C02NCQ9483C/p1708019935717339?thread_ts=1708018186.785359&cid=C02NCQ9483C)