Updates the Slack integration page to not describe adding a stream
or topic parameter to the URL query since that's not supported by
the current integration implementation.
Updates the Slack-compatible webhook integration page to have the
extra notes about the integration at the top of the page. Also,
removes the reference to a screenshot of the webhook since there
isn't one.
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>