MAINT: Update the documentation Makefile

[charris]

This does several things

• Uses a virtual environment for the document 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.

EDIT: There are a number of other targets for making helpfiles, coverage, etc. Some modification is needed to run them in the virtual environment. What should we do with those? Are they used downstream? One workaround would be to first run make docenv; source docenv/bin/activate and proceed as before. This could also be used for the two commands (make dist and make merge-doc) that we currently use. I'm thinking this should this be the standard method and the easiest for possible downstream users.

[melissawm]

Should https://numpy.org/doc/stable/dev/releasing.html be updated accordingly?

[charris]

Should https://numpy.org/doc/stable/dev/releasing.html be updated accordingly?

Yep, also the walkthrough, which needs other changes as well. The big change is the missing pdf files, make dist or merge-doc will otherwise work as before, just a bit better, so no changes are needed respecting those commands. We haven't used paver for building docs since forever, so that bit can also be deleted from the release documentation.

[charris]

I think PDF needs changing to HTML+zip for 1.24:

Download documentation: PDF Version | Historical versions of documentation

I'm not sure where it resides, or what the best way forward is there.

EDIT: It's in doc/source/index.rst, but is just a link that opens the PDF. That isn't quite the same as a download.

[charris]

We don't seem to have a good explanation of local wheel builds. Maybe just python setup.py bdist_wheel together with OpenBLAS should be sufficient?

[charris]

Self merging. @melissawm There are three files that should probably be edited so that they fit together in the documentation.

1. HOWTO_RELEASE.rst
2. BRANCH_WALKTHROUGH.rst
3. RELEASE_WALKTHROUGH.rst

I cut out a fair amount of the HOWTO_RELEASE.rst file as duplicated in the walkthrough.

[melissawm]

Thanks, @charris !