doc: Move output examples from "examples" to "pspp-figures".

doc: Move output examples from "examples" to "pspp-figures".

The Texinfo manual suggests this convention:

     If you want to install image files for use by Info readers too, we
  recommend putting them in a subdirectory like 'FOO-figures' for a
  package FOO.  Copying the files into '$(infodir)/FOO-figures/' should be
  done in your 'Makefile'.

Although it suggests this convention for the installed Info, it helps to
use the convention for the source and build directories, because "makeinfo"
looks for the files at Info build time.