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.