Added a mention about the `prop` thesis type to the Faculty of Informatics (MU Brno, Czech Republic) user guide.

This conditional determines, whether the scanned assignment
specified by the `assignment` key is going to be hidden in the
digital version of the document. It is by default, but some style
files (`/mu/sci`) switch this, so that the scanned assignment is
included in the digital version.

The output of \thesis@block@assignment and \thesis@@{assignment} now
depends on the value of the new \ifthesis@digital@ conditional.