Added Debugging recipe.
Fixed Deployment > Backend > Google Cloud - App Engine docs.
Fixed the firebase-tools install script on Deployment > Frontend > Firebase Hosting.
Added Rate-Limiting to the architecture section.
Added the unique validation to the Modeling section.
Added the Deployment > Backend > Google Cloud - App Engine > Build step.
Added the Setup > Backend > Creating the database section.
Improved Setup > Emails with SendGrid explanation about the template creation.