Several integration docs instruct the user to create a bot, but don't
specify that the type of bot should be "Incoming webhook".
Renames create-a-bot.md -> create-an-incoming-webhook.md for clarity,
and replaces all incomplete instructions with this macro.
Renames bot_types.png -> bot_types_incoming_webhook.png and updates
the image with a screenshot of the latest UI.
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>
David Rosa
Anders Kaseorg
Tim Abbott
Mayank Madan