docs: refactor getting-started codelab to be based on sample app

[zspriggs]

The basics

• I validated my changes

The details

Proposed Changes

• Refactors the getting-started codelab to be based on the existing sample app rather than the custom music-maker app.
• The content has remained the same where possible, with slightly more elaboration throughout on what the copy & paste code is actually doing.
• The codelab goes over largely the same topics, but focuses more on important Blockly concepts than configuring a specific page.
• Adds the sample app README to the getting started docs, so that the existence of the sample app is more visible and the overview can be referenced from the codelabs.

[gonfunko]

This also fixes RaspberryPiFoundation/blockly-samples#1752