docs: move using kuadrant from guides into readme for guides

[KevFan]

Description

Closes: #519

Move out the "Using Kuadrant" from the user guides to keep the user guides focused on AuthConfig. Instead a README.md is introduced describing Kuadrant AuthPolicy as a consumer of the AuthConfig

[guicassolato]

While revisiting this, I had doubts about centralising the disclaimer for Kuadrant users in a separate doc that one could easily overlook. However, I now think it's worthwhile the trade-offs, which I'd probably summarise as follows:

✅ Pros:

• Reduced noise for direct Authorino users - The guides become cleaner and more focused on their core purpose (demonstrating AuthConfig features). The current boxes interrupt the flow twice per guide.
• Better discoverability of limitations - The centralised README explicitly lists features NOT available via Kuadrant (Sharding, Host Override), which is harder to communicate in repeated inline boxes.
• Single source of truth - Information about Kuadrant integration lives in one place, making it easier to:
  • Keep consistent across all guides
  • Update when the relationship changes
  • Add other AuthConfig consumers in the future
• Improved maintenance - Fewer places to update when Kuadrant's API or relationship changes.
• Scalability - If other projects become AuthConfig consumers – at the same as Kuadrant, we add one section to README.md instead of modifying tens of other files.

⚠️ Cons:

• Discoverability (the other side) - Kuadrant users following a specific guide might miss the README and get confused when instructions mention AuthConfig instead of AuthPolicy.
• Context switch - Users need to jump to another doc to understand the Kuadrant-specific adaptations. The inline boxes provided just-in-time reminders.
• First-time user experience - Someone coming from Kuadrant docs might land on a specific guide via search/link and not see the README context.

If I may suggest only a couple of enhancements before we go ahead and merge this PR:

1. Keep a single brief callout at the top of the guides:

   Using AuthConfig via Kuadrant or other tools? See the AuthConfig Consumers guide for adaptation notes.

2. Ensure the README is prominently featured in the navigation – E.g., add a link to it from the main README of the repo and user guide index page.

If there's anybody interested in taking ownership of this and implement these suggestions, I'll be glad to approve them.