Updates `git-webhook-url-with-branches.md` (and two files that use
that file as an include link) for some of the follow-ups from #22315
to the Markdown parser. With this fix, all integrations docs that
reference this file as an include link should render the url as a
div element with `.codehilite` class.
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>