epic: Improve the auth docs

[Swiftaxe]

Summary of the issue

Investigate and improve the authentication docs to:

• Clarify
• Simplify
• Add missing steps and code examples
• Address coverage gaps and common questions

[marcelomendoncasoares]

If you're mapping everything, there are three missing pieces that we didn't have the proper time to add yet regarding custom extensions on the auth docs. Two are from the issue (serverpod/serverpod#4212):

• Implementing custom token provider
• Implementing custom identity provider

And the third is about other utilities that we have for implementing custom identity providers:

• A pre-built rate limiter (RateLimitedRequestAttemptUtil).
• A secret challenge issuer/manager (SecretChallengeUtil).