docs: add development procedures

[Regaez]

Document development procedures, such as:

• commit naming conventions, zappr config
• issue/PR process/requirements, and project board
• documentation, e.g. specification, postman collection
• testing framework/setup
• how to debug with vscode/xdebug
• adding to project's Grav site user folder
• how to update grav version in development environment
  • adjust dockerfile
  • bump composer dependency

This should assist with knowledge transfer for anyone looking to contribute to the plugin's development.