Fix the broken docs tox environment

[chala2001]

What type of PR is this?

/kind bug
/kind cleanup

What this PR does / why we need it:

tox -e docs cannot run. It calls python setup.py build_sphinx, and that command
came from sphinx.setup_command, which Sphinx removed in 7.0. test-requirements.txt
pins sphinx>=8.1.3, so on a current checkout:

$ python setup.py build_sphinx
error: invalid command 'build_sphinx'

Fixing the command alone is not enough. doc/source/conf.py imports recommonmark
and lists it as an extension, and recommonmark appears in none of the four
requirements files the tox env already installs - it is only in
doc/requirements-docs.txt, which the env never pulls in. So the environment would
still have failed while loading conf.py.

This PR points the env at the build doc/README.md documents (make html, which is
sphinx-apidoc then sphinx-build) and adds doc/requirements-docs.txt to its deps.
After the change:

docs: OK (13.89 seconds)
congratulations :)

Two related bits of tidying that belong to the same story:

• setup.cfg contained [build_sphinx] and [upload_sphinx] and nothing else. Both
  are settings for the command Sphinx removed, so the file goes with them. The only
  other setup.cfg references in the tree are to the generated kubernetes/setup.cfg,
  which is a flake8 config and is untouched.
• envlist still named py3{6,7,8,9}. setup.py sets python_requires='>=3.10' and
  CI tests 3.10 through 3.14, so all four named environments are below the supported
  floor.

Which issue(s) this PR fixes:

No open issue - I hit this while looking at the packaging config in #2687 and #2688.
Happy to file one first if you would prefer that.

Special notes for your reviewer:

I verified the docs toolchain itself is healthy under Sphinx 8.1.3 - sphinx-build
loads conf.py with recommonmark and sphinx_markdown_tables and reports
build succeeded, and sphinx-apidoc generates all 1,810 module pages. I did not
run the full 1,800-module render to completion; that build is memory-hungry and my
machine ran out of RAM part way through writing HTML. Everything this PR claims is
about the tox environment resolving and invoking the right command, which is what
the docs: OK above shows.

Independent of #2688 - different branch, no shared files.

Does this PR introduce a user-facing change?

NONE

[kubernetes-prow]

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This pull-request has been approved by: chala2001
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