`$(txtdir)` -> `$(docdir)`

`$(txtdir)` -> `$(docdir)`

* All documentation directories should be a function of `$(docdir)`, and
  not `$(datadir)`, which is to be used for files that the program
  actually needs at runtime.
* Also amend the debian install rules, since documentation is now
  natively installed into `$(docdir)` == `usr/share/doc/graphviz`.