Issue description

Documentation pages currently hide the "sidebar" in responsive mode (eg small screen sizes), which means the navigation isn't visible on mobiles, making the docs unusable. The issue is with an over-zealous display: none at

-- PR is incoming to remove this line which fixes the issue.

It's not as obvious on the homepage because there's lots of navigation links in the content, but it's clear on a sub-page like https://pipenv.readthedocs.io/en/latest/dev/philosophy/ (seen on a small screen):

Expected result

The page should have the "sidebar" navigation at the bottom:

Actual result

As per the expected result.

Steps to replicate

1. Load a docs URL https://pipenv.readthedocs.io/en/latest/dev/philosophy/ either on a mobile device or just use responsive design mode in browser.
2. Note that the navigation isn't visible.