Summary as common Section

While the description describes the document to show if the content is interesting for the reader, we need another common section that collects the essential information, something like an abstract. This makes it easier for authors to start with some basic information and get them transcluded in overview documents.

Until now we often used the Description Section for this, but this is usually not the intention of this section.

This will also change the Content Section of the Documentation Module Doctype. The main benefit of this approach is: the section heading "Summary" is more natural for readers than "Content" (which just makes it clear that this content is for reuse.

Examine, if we should add a "Summary Section" to all our doctypes.