Allow for filtering coding guidelines by chapter

Objective: We want to allow for showing only coding guidelines that come from a particular chapter.

We believe the following needs to be done
- In the XML to Markdown converter wrap each guideline in a `span` with chapter number (extracted from key attribute in XML) with the chapter number as a css class.
- Inject a block of JavaScript and markup to allow for toggling the visibility of items with that class.

Start at the XML-to-Markdown converter and inspect how each guideline's key attribute is parsed. The work is done when guidelines carry chapter-number CSS classes and the rendered output includes controls that toggle visibility by chapter; no specific file or test is named in the issue.

Vom Indexierungsmodell aus dem Issue-Text verfasst.