RHCLOUD-47190: adds an end-to-end integration with Kessel tutorial

[tonytheleg]

For RHCLOUD-47190, the goal was to create a tutorial that could target developers who have an existing service and need to add Kessel authorization to it. It expands a bit more on the quick start and hopefully functions as a way to build users chops with Kessel

Changes:

• Update Quick Start to add time estimate and "What you learned" summary (Diataxis alignment)
• Adds new "Add Kessel to an existing application" tutorial under Start Here
  • New tutorial cross-references how-to guides without duplicating content and expands on whats learned in quick start
  • the goal is to focus on integration seams (SDK client setup, reporting at CRUD boundaries, permission checking via interceptors) instead of duplicating the Quick Start's walkthrough
  • to drive learning, it doesnt outright provide examples of the schema files or code so that users can try without assistance BUT examples are provided and linked in case they get stuck