Text tweaks

Add more explanatory text, links, standardise on 'gu:who'

Created the https://github.com/gu-who-demo-org/people repo as a public example of what the repo will look like (obviously, normally it would be private!).

How do we "gu:who?" "GU-WHO?" "GU:Who?"
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Use the decorative '_gu:who?_' form only if name is *standing alone*, AND can be presented in italics. Otherwise just '_gu:who_', as the question mark is way too distracting.

Also, use 'organisation' in preference to 'org' to avoid confusion with domain names, as @gidsg pointed out.

Consistently use brand-capitalisation of 'GitHub'.