docs - improve combination of descriptions from different sources

it should be easier to combine external and exported descriptions (and maybe other parts of the documentation).

some files and tags could be automatically included into descriptions

• use optional includes include::test_gibtesnicht.adoc[tag=ccc,opts=optional]
• find ways to add descriptions not only per page but also for sub elements (SSAS measures)
• use tags in this includes to exlude exported tescriptions, for example 'exclude-description', 'exclude-description- measure-xyz'