doc/Makefile to replace doc/process?

@hjoliver, I thought it would be desirable to replace the `doc/process` and `doc/cleanup.sh` scripts with a single `Makefile`. There are several advantages:

* People are more familiar with typing `(cd doc; make)` than typing `doc/process`.
  * Also `(cd doc; make html)`, `(cd doc; make pdf)`, etc.
  * Can also do `make -C doc`.
* Ditto for `(cd doc; make clean)` c.f. `doc/cleanup.sh`.
* (The Makefile has fewer lines than the 2 original scripts.)
* (Unfortunately, I can't get it to do multi-process `make` because the single and multi-page HTML generation would clash.)

What do you think?