merge doc/pddp with pd/doc/5.reference

Right now the build script pulls from both pd/doc/5.reference and doc/pddp for what ends up in $LIBDIR/doc/5.reference. IIUC The build script overwrites the pd.doc/5.reference stuff with the contents of doc/pddp.

So we need to:

1. do a git log pd/doc/5.reference to see what changes we've made to the core Pd docs
2. make sure none of those changes are currently getting overwritten by doc/pddp contents when building Pd
3. copy contents of doc/pddp into pd/doc/5.reference
4. remove all references to doc/pddp in the monstrosity of makefiles
5. test
6. git rm -rf doc/pddp