Post-Documentation Tweaks

This PR is a result of changes that were deemed necessary due to shortcomings found while building our internal documentation.

As necessary changes come up, I am going to add those changes to this PR.

Status: **Open for Visibility**
Reviewers: @kpeatt @ry5n 
JIRA: n/a

## Changes
- Add a link to csswizardry.com that talks about keeping your CSS selectors short and low specificity
- Add another link to an article on Smashing Mag about how to calculate specificity
- Update section on `@extend` to actually justify why we don't like it
- as well as include some legitimate use cases for using `@extend`
- Update the section on "nest only when necessary" with use cases and strategies

## Notes
- This PR is probably going to evolve a bit over time