docs: add example authenticating against 3rd-party APIs

[falcowinkler]

☕️ Reasoning

A much requested example is how to use nextjs to integrate with an external backend. This PR adds an example utilizing our test keycloak instance, of how to integrate with the example backends (should be made public before merge). It will only show to the user if keycloak is actually used.

🧢 Checklist

• [ ] Documentation
• [ ] Tests
• [ ] Ready to be merged

🎫 Affected issues

📌 Resources

• Security guidelines
• Contributing guidelines
• Code of conduct
• Contributing to Open Source

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Name	Status	Preview	Comments	Updated (UTC)
auth-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	May 2, 2024 8:06pm

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| Name | Status | Preview | Comments | Updated (UTC) | | :--- | :----- | :------ | :------- | :------ | | **next-auth-docs** | ⬜️ Ignored ([Inspect](https://vercel.com/authjs/next-auth-docs/4BfZhwrRwhQKhz17UzK7AJN77sqW)) | [Visit Preview](https://next-auth-docs-git-fork-falcowinkler-main-authjs.vercel.app) | | May 2, 2024 8:06pm |

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[ndom91]

Awesome, thanks! Would be nice to have soemthing in that doc guide as well as a part of this PR :pray:

EDIT: Hope yuo don't mind, I added a README to the third-party-backends repo you mentioned also linking to this PR and the (soon to come) guide