Make chapter abstracts mandatory, restrict them to one para with 150 characters

[ghost]

Description

Search engines like to show short descriptions, ideally 70-150 characters (meta description tag).

We'd like to not introduce too many different description elements, to avoid duplicating work here.
So, the idea we had in Stylewich was to use the <abstract/> element to do both: on-page abstract and meta description.
The stylesheets are already set up to funnel <abstract/> text into the meta description anyway, so Geekodoc 2 is the last piece of the puzzle to ensure some quality here.

Expected Behavior

So what we want in Geekodoc 2 only is this:

• make abstract mandatory within set, book, article, preface, part, chapter, appendix and topic
• at least consider making abstract mandatory within top-level sections like sect1, article/section and (preface|chapter|appendix)/section
• enforce that there is a single para with only simple inline styling that converts well to plain-text within abstracts
• enforce that pure text content is at least 70 characters and at most 150 characters long -- I hope that's possible in RNG... You could definitely define a regex such as .{70,150} but I am not sure if that would run on pure text content or all content of the abstract.

cf. SUSE/doc-styleguide#182

[tomschr]

We shouldn't restrict an abstract, it should be done with meta information. This is covered in #98 so this will be obsolete (and a wontfix issue).