Reorganize pip's docs folder

[pradyunsg]

This breaks up pip's docs folder into 2 folders: html/ and man/. They hold exactly what you expect them to.

It ensures that when HTML output is generated, it doesn't include man pages: https://pip.pypa.io/en/stable/man/pip/. (this has been the case since they were added and it's not-nice)

---

The conf.py file is updated to make the required changes and stays in the same directory, while the sources move. This requires passing -c to sphinx-build, so I'll check if RTD would be happy about it.

[pradyunsg]

This requires one change to RTD:

[pradyunsg]

Before this PR:

docs
├── __init__.py
├── conf.py
├── [html docs sources]
├── pipext.py
└── man
   └── [man pages sources]

After this PR:

docs
├── html
│   ├── conf.py
|   └── [html docs sources]
├── man
|   └── [man pages sources]
└── pip_sphinxext.py

[pradyunsg]

@pypa/pip-committers I'm willing to go ahead and merge this -- any thoughts?

[pradyunsg]

I've gone ahead and merged this since I doubt anyone would really mind this change. I'll be happy to make additional changes in follow up PRs. :)