Massively reduce the amount of text, cutting it down to what is needed

Massively reduce the amount of text, cutting it down to what is needed
to understand existing man(7) code and deleting parts that would only
be useful for writing new documents, which we strongly discourage:

* Delete the MANUAL STRUCTURE section which merely duplicates mdoc(7).
* Delete internal cross references only useful for writing new code.
* Delete many instances of "included only for compatibility" as the
  whole language is only provided for compatibility.
* Fix a few minor errors and omissions.