Restructure the reference documentation:
- draw NAPI docs out of core reference
  (because of frozen status, and for clearer overall structure)
- explain basics of base classes and applicn defs before introducing NXDL
- move class definition chapters to second level
  (for easier browsing in HTML, and for clearer structure in the PDF version)
Improve verbal embedding of link.
Intro to NeXus definitions into one HTML page.
Revise the sections
  2.1. Introduction to NeXus definitions
  2.1.1. Overview of NeXus definitions
to get rid of historic ballast and duplications.
Trying to make sense of the old chapter "Basic structure of the class".
Remove example in obsolete physical data format (XML).
Updated NAPI status.
Conclude my first round of contributions to the manual.