Update Slack notifications create webhook link to Slack docs
What does this MR do and why?
Describe in detail what your merge request does and why.
The old link pointed to a general instance of Slack (my.slack.com) that wasn't really helpful. I'm replacing with a link to the Slack documentation that outlines how to create a Slack app if it hasn't been created correctly and how to create the incoming webhook from there. The rest of our instructions should still apply.
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