Add missing required sections to the UI and CLI documentation

The Google documentation [states](https://cloud.google.com/marketplace/docs/partners/kubernetes-solutions/create-app-package#user-guide) that the user guide should contain certain sections. In the following list, I have checked the items that we already have:

- [x] Overview
- [x] One-time setup
- [x] Installation
- [x] Basic Usage (I added a section where I explain how to create a certificate)
- [ ] Back up and restore
- [ ] Image updates
- [ ] Scaling
- [ ] Deletion

This issue aims at reminding us to add the 4 missing sections both for the UI and the CLI instructions.