Updates text of generate-webhook-url-basic.md to replace the use of
the "Create the channel" step.
Removes {!create-channel.md!} from all the integrations docs that
also include {!generate-webhook-url-basic.md!}.
Fixes#32269.
Note about the documentation: There are currently two "Save changes"
buttons on the Airbyte "Notifications" settings page, so the
instructions specify which one to use for clarity.