This commit carves out the overview for incoming webhooks and
moves it to its own file. This is a much better way to structure
these docs.
This is a quick follow-up to Tim Abbott's comment on #9592.
I only renamed references that I thought were absolutely necessary
and only if the resulting sentence structure wasn't awkward.
If the renaming resulted in awkward structure, I replaced the term
"webhook" with "integration" (but only in some very obvious cases).
Fixes#9500.
Tim Abbott
Rishi Gupta
Eeshan Garg
Viraat Chandra
derAnfaenger