Initial refactoring of htmlnotebook.txt into 3 parts.

Renamed htmlnotebook.txt to notebook.txt. 79-char width hard wrapping imposed. Rearrangment of order and level within notebook.txt. Almost no text modified.
txt/rst in notebook docs
fix a few markup issues and inaccuracies
Merge Pull Request #3772

from dpsanders/notebook_docs_refactored

closes #3772