Kludge for 'make docs' to produce pretty nim manuals

why:
  Some helper file will not be generated by the nim document gereator,
  so they have been stashed from a later nim version to be provided when
  missing.

  This problem was known with an earlier nim version (see here
  nim-lang/Nim#8952) but was reported solved.
  Maybe we need a second look into that.