makefile: fix building docs.zip, and rename the target to docs-zip

Makefile's docs target is currently broken in several ways:

- Make refuses to run that target when `docs` directory exists (it does).
- Every line is executed in its own shell session, so `cd` statements have no effect and the whole tree is zipped.

This PR fixes that. You may also consider adding `makefile` and `tox.in` to `MANIFEST.in`.