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.
The "subdomain" label is redundant, to the extent it's even
accurate -- this is really just the URL we want to display,
which may or may not involve a subdomain. Similarly "external".
The former `external_api_path_subdomain` was never a path -- it's a
host, followed by a path, which together form a scheme-relative URL.
I'm not quite convinced that value is actually the right thing in
2 of the 3 places we use it, but fixing that can start by giving an
accurate name to the thing we have.
Satyam Bansal
David Rosa
Anders Kaseorg
Adam Birds
Stefan Weil
Tommy Ip
Greg Price
Eeshan Garg