docs: refactor validation and warnings codelab to use sample app

[zspriggs]

The basics

• I validated my changes

The details

Proposed Changes

The main changes with this refactor are deleting the starter code, updating the complete code to reflect the new starter code (aka the sample app), and updating the setup instructions accordingly.

This refactor also includes a couple of extra changes:

• Moved the block creation step to its own page of the codelab, rather than being part of the Setup step. This reflects the pattern in other codelabs of having a smaller Setup step that just goes over setup without including code.
• Updated screenshots to reflect the blocks that are actually available in the sample app