In Doc/Makefile, all of the build rules should be dependent on the existence of a virtual environment. I could see this being controversial, because folks who have these tools installed elsewhere might prefer not to have a venv made for them, but my personal workflow is to strive to keep my system site-packages folder as empty as possible and to use virtual environments frequently. In any case, I'm interested to hear what we think.

Momentarily, I will attach a PR where I went ahead and made these changes. Here is a summary of the changes:

- venv rule is now conditional, and only does anything if $VENVDIR does 
  not exist
- add rule "clean-venv". This can be removed – what do you all think?
- build rule is dependent on venv
- as a result, rules dependent on build are dependent on venv
  - html
  - latex
  - etc.
- update Doc/README.rst appropriately. Now users only need to type 
  ``make html`` – nice!

Let me know what you think. I may have a blind spot to others' workflows! Also, I'm not a Windows user, so I have no insight as to whether ``make.bat`` needs to be updated as well.