Add a CSS codelab

[rpbourret]

The basics

• [x] I validated my changes

The details

Resolves

Adds a CSS codelab.

Proposed Changes

Adds a CSS codelab in the form of a Markdown file, PNG screenshots, and starter and completed code.

Reason for Changes

We need a CSS codelab.

Test Coverage

Manually tested.

Documentation

Included.

Additional Information

[BenHenning]

I think @maribethb is planning to look at this, so de-assigning myself unless I'm told that I need to review this.