MAINT: Update the documentation Makefile

This does several things

- Uses a virtual environment for the documentent build. This allows
  installing the proper NumPy version with pip and works with all
  recent Python versions.
- Installs the dependencies current in the {doc|test}requirements files.
  That makes it easier to insure that the proper versions are used.
- Removes the latex/pdf builds. Those builds are currently buggy and
  hard to maintain and we have chosen to remove the pdf documents until
  that is fixed. Removing them cleans up the file to some extent.

This all makes it easier to document how to build the docs.  I was
editing the Makefile on the fly before this, and that sort of knowledge
is tricky to pass on to new maintainers.

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