Make `example usage` self contained

[aliphys]

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What is the problem?

• The current example under https://github.com/arduino/compile-sketches#example-usage is not a self contained example. This increases the barrier of entry for newcomers to the powerful arduino/compile-sketches GitHub Action
• As an example, I was under the impression that the checkout action was included as part of the arduino/compile-sketch GitHub action. However, checking out the repo is a required step prior to compiling the sketches.
• There is a helpful blogpost here: https://blog.arduino.cc/2021/04/09/test-your-arduino-projects-with-github-actions/. However, it has too much information leading to information overload.

What This PR Changes

• I have elaborated upon the example to make it self contained. This includes:
  • Fully defining the jobs: entry, alongside the on: and name: items
  • Adding comments describing what the .yml file does, without assuming prior knowledge with GitHub Actions
  • Enabling the workflow_dispatch: option, so users can manually run the workflow

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Note: I have a repo here, where the .yml file is fully tested alongside example sketches.