Updated Auth Getting started Guide

[awanlin]

Hey, I just made a Pull Request!

Updated Auth Getting started Guide as there were some missing parts:

• Installing the backend auth provider
• Adding signIn.resolvers to the app-config.yaml
• Adding a User to the Catalog

:heavy_check_mark: Checklist

• [ ] A changeset describing the change and affected packages. (more info)
• [x] Added or updated documentation
• [ ] Tests for new functionality and regression tests for bug fixes
• [ ] Screenshots attached (for UI changes)
• [x] All your commits have a Signed-off-by line in the message. (more info)