The integrations - GoSquared, Greenhouse, OpsGenie and Zendesk - have
been using `000.png` as their example screenshots, renamed them to
match the default name `001.png`, to avoid passing them as arguments.
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.
markdown-include is GPL licensed.
Also, rewrite it as a block processor, so that it works correctly
inside indented blocks.
Signed-off-by: Anders Kaseorg <anders@zulip.com>
