Add troubleshooting info from @adamvb

[misterpantz]

Description

@AdamVB created this PR: #497
and wrote: Providing a simple curl command helps customers test that at least the MS Teams webhook receiver is configured correctly.
Also highlighting some easy mistakes about private channels and how to fix it.

I edited that PR but something is wonky with it so I'm just making a new one. 🤷

Related issue

none

Types of changes

• Functional documentation bug fix (i.e., broken link or some other busted behavior)
• New functional doc capabilities (i.e., filter search results)
• New content
• Revision to existing content
• Chore (non-breaking change that does not add functionality or fix an issue)

Checklist

• I have read the README document about contributing to this repo.
• I have tested my changes locally and there are no issues.
• All commits are signed.

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