doc: added prepdoc.sh script for simple working on documentation

doc: added prepdoc.sh script for simple working on documentation

I added the prepdoc.sh script to allow to work on the documentation
without having the full build environment. You can just checkout
from git and then run

  cd pspp/doc
  ./prepdoc.sh
  makeinfo --html pspp.texi
  open ./pspp/index.html

Maybe it is then easier for contributors to work on the documentation
without having to run Smake and configure with all the required
packages. prepdoc.sh will generate some dummy required texi files
which are normally generated during the build process.