docs: refactor toolbox codelab to use sample app

[zspriggs]

The basics

• I validated my changes

The details

Proposed Changes

Refactors the "Customizing a Blockly toolbox" codelab to use the sample app as starter code, rather than custom starter code. This is one of the more extensive refactors, because the original codelab used an XML toolbox, but the sample app uses a JSON toolbox, so the codelab had to be changed accordingly in a few different places.

There are also a few other changes in this refactor that were made for consistency and/or clarity:

• Moved setup of custom class from Setup step to new Make a Custom Category step. This makes this codelab more consistent with other codelabs, which have very minimal Setup steps that don't include any coding.
• Added call to super() in setSelected() override and explained why. The old codelab set the ARIA state in the override without really explaining it, so I changed it to use super for slightly better form and added an explanation.
• Changed Blockly logo to toolbox emoji. Previously the Blockly logo svg was shipped with the starter code, but since the starter code is now the sample app, I switched it to a toolbox emoji so that users don't have to deal with a separate download.