manpage documentation is missing all subcommands

[eli-schwartz]

Initially reported in Arch Linux as: https://bugs.archlinux.org/task/63223

To reproduce:

git clone https://github.com/pypa/pip/
cd pip/docs
PYTHONPATH=$PWD/../src/ sphinx-build -W -b man -d build/doctrees/man man build/man -c html

Look in build/man and you will see only one manpage: pip.1

Really quick reproducer: look at a recent Travis CI build for the TOXENV=docs results, for example https://travis-ci.org/pypa/pip/jobs/559973823#L388, and see only one file being written out.

Expectation: There should be lots of manpages, one for each pip subcommand, and linux distro packages which install the docs/build/man/ directory to /usr/share/man/man1/ should be able to read all about pip's many excellent features in their offline documentation reader.

The cause of this breakage is #5724, which reorganized the conf.py layout due to readthedocs/readthedocs.org#1543 but did not adapt the somewhat hacky code to automatically add new entries.

[pradyunsg]

Makes sense. Happy to merge your fix conditional to a minor nitpick. 🙃