Own pages for builtin functions - Basics

https://trello.com/card/own-page-for-each-built-in-function-function-group/4ff142712c442e040466599d/24

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Contents of this pull request:

 - General cleanup of page "functions.htm" concerning format unification of functions, which do already have a own page
 - StrLen() is documented on page StringLen.htm (including adaption of links ...)