Add pre-configured Gitpod to make it easier for contributors to contribute

[tylers-username]

Features

• Adds Gitpod development environment
• Ready to run tests
• Environment configured on deploy
  • e.g. composer install
  • semi-auto-configured PLATFORM_CLI_TOKEN. Contributor prompted to provide token the first time they launch the Gitpod.
• Adds Gitpod documentation
• Adds Gitpod dockerfile- only because Gitpod defaults to PHP 8.1 but we need PHP 7.4