docs: refactor css codelab to use sample app

[zspriggs]

The basics

• I validated my changes

The details

Proposed Changes

Refactors the CSS codelab to use the sample app.

Main changes:

• Removed starter code since the sample app is the new starter code
• update complete code to be based off sample app
• change setup instructions accordingly

For this codelab, I also:

• updated codelab to use <Image> component rather than pure markdown to conform with other codelabs
• re-shot screenshots to reflect the exact app the user will see (thrasos renderer, code output window on the left, etc)