feat: document and refactor GitHub example for readability

[yuhgto]

Made some changes to make the Github example easier to understand and extend to another API.

The changes:

• Split the custom trigger and custom action logic into their own files. Most developers start with either a trigger or action but not both.
• Documented all controller functions and most services.
• Explained what each DB table does.
• Cleaned up the Readme a bit

The documentation is written in JSDoc format so it will get picked up by most IDEs:

Let me know if you have any questions! :)