AsciiDoc: Add folding for sections

Summary:
As D18516 is commited now, we can add sound folding for sections.

• Add contexts to support section folding in highlighting definition asciidoc.xml.
• Update and improve test file asciidoc.adoc.
• Update created .fold, .html and .ref files.

Reviewers: Framework: Syntax Highlighting, dhaumann

Reviewed By: Framework: Syntax Highlighting, dhaumann

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D18895