Reorganize and document configuration ("external text") file

[rod-glover]

External text now delivers generic configuration data objects, not just text rendered from Markdown.

The external text file (needs renaming) is a bit disorganized following rapid development in early 2020. Mostly the problem is that related information is scattered in several different places. Also renaming some fields might help.

It also needs to be documented; the best place is to do it inline within the external text file using YAML comment lines. A paragraph in the project README should point the user at this documentation.