Top-level navigation hard to read on mobile widths.

[jobara]

Describe the bug

The Infusion Docs site lacks any responsive design making it hard to use on mobile devices.
For example, resizing the browser causes the top-level navigation to wrap onto white space causing the text to be illegible.

To reproduce

Steps to reproduce the behavior:

1. Go to Infusion-Docs
2. Set the width of the browser to something narrow similar to what a mobile device might display
3. Notice the top-level navigation wraps over the content background making it hard to read.

Expected behavior

In mobile widths we should user a disclosure navigation menu.

Screenshots

Additional context or notes

Originally filed as FLUID-6376. Mothballed PR #182.