Add demo / screenshots

[jtagcat]

When comparing many wikis, one can't even start to compare, without first having to deploy the solution.

[jgm]

We used to run a demo. Sadly, some very persistent spammers made it impossible to keep it up. A screenshot could be a possibility, or maybe a demo site that erases its data daily.

[jtagcat]

One thing what many wikis do is to have a wiki about the wiki (docs).

[jtagcat]

Making it dockerized, would add a feature of being dockerized, but also easy (daily) rollbacks.

[jgm]

If you want to see a demo, just search google for "powered by gitit" and you'll several live sites, including https://pari.math.u-bordeaux.fr/wiki/ http://wiki.wcaleb.rice.edu https://wiki.couchcooperative.org.