Fix pip config docstring to render correctly in docs

[deveshks]

Fixes and closes #8072

The formatting is picked from https://github.com/pypa/pip/blob/master/src/pip/_internal/commands/download.py#L19 and the docs now look like

They pip config doc can also do with some examples as well, but I can perhaps make a separate PR for that.

$ pip config set site.proxy 'http://localhost:3128'
Writing to ~/.config/pip/pip.conf
$ pip config get site.proxy
http://localhost:3128
$ pip config list
site.proxy='http://localhost:3128'

Edit: Seems like the docstring indentation was changed in bbd4398 but that didn't correct the rendering

[deveshks]

If there are no more changes needed here, may I get this PR merged?

[pradyunsg]

We should probably do this to pip cache as well!

[deveshks]

We should probably do this to pip cache as well!

Done in #8313