Add demonstration of arduino/compile-sketches action

[per1234]

The GitHub Actions workflow provides a minimal demonstration of using the arduino/compile-sketches action for a "smoke test" sketch compilation CI check.

Although it provides a higher level interface to the user, handling all the typical requirements for "smoke test" sketch compilation CI applications, the foundation of the arduino/compile-sketches action is still very much Arduino CLI.

This demonstration is stored in a dedicated compile-sketches-demo branch, so it has no impact on the demonstration of the more low level arduino/setup-arduino-cli action which is stored in the master branch.