People might just copy-paste it, while there's usually no reason
to enable this by default for new installations; specially not as
a QucikStart.

See https://github.com/simplesamlphp/simplesamlphp-module-casserver

Fixes #873

SHA-1 is still supported but needs to be configured explicitly if you need it.

Per suggestion from @tvdijen

Add some basic documentation on how the new Twig templating system works.