Add topic(s) on DITA features used in docs

[infotexture]

The docs should be extended with a dedicated topic or set of topics that describes how the DITA-OT documentation uses certain DITA and DITA-OT features, such as:

• profiling and filtering
• keys and key references
• subjectScheme classification for controlling available attributes
• automatic generation of topic files based on the content of installed plug-ins
• conref push to inject additional descriptions into generated topics for messages, parameters & extension points

Wherever possible, these topics should use <coderef> elements to link directly to live source files rather than copying code (see #49).