XQuery Style

take over some best practices from fontane-notizbuecher/SADE#21 (comment 34420)

with removed overlap to xqdocs and removed specific things from the original repo, we have the following to add:

• use element constructors when creating a dynamic output. use the direct notation for rather static content, like icons from font-awesome or the like. golden rule: a direct notation may contain a maximum of one single simple XQuery expression inside with { some very simple query }. Anything else must be written in constructors!
• use minified variable names only at control variables - anything else must be readable and understandable.