## What changes were proposed in this pull request?
A minor CSS style change to make Left navigation bar stay fixed with respect to scrolling, it improves usability of the docs.

## How was this patch tested?
It was tested on both, firefox and chrome.
### Before
![a2](https://user-images.githubusercontent.com/992952/33004206-6acf9fc0-cde5-11e7-9070-02f26f7899b0.gif)

### After
![a1](https://user-images.githubusercontent.com/992952/33004205-69b27798-cde5-11e7-8002-509b29786b37.gif)

Author: Prashant Sharma <prashsh1@in.ibm.com>

Closes #19785 from ScrapCodes/doc/css.