Rework the Slack integration documentation.
Review Request #9802 — Created March 17, 2018 and submitted — Latest diff uploaded
This updates the Slack documentation to better help people get set up
with configuration the integration. The paragraph of explanatory text
has been split into a step-by-step guide with screenshots, and includes
information on how to configure an Incoming WebHook in Slack (which is
not entirely obvious normally).
Built the docs, checking for build errors and typos. Also checked that
the screenshots appeared correctly and were pixel-aligned across normal
and Retina displays.