Doc: ensure all level 3 and 4 headings have nodes (2967)

Doc: ensure all level 3 and 4 headings have nodes (2967)

  Without them the sections cannot be referenced,
  and links in the left panel in html and entries in
  the index do not link to the correct section.

  Correct more level 5 headings.

  Usage manual is not included.

  Complete the set of files in Documentation/nl/learning
  and Documentation/hu/learning with the English versions.
  This is needed to avoid errors in make doc following
  the change in node names.  Fixes Issue 2980.