docs: add Auth Hooks guide with beforeLogin hook documentation

[k1LoW]

This pull request introduces documentation for Auth Hooks, a new feature that allows custom logic to be executed during authentication flows. The main addition is a dedicated guide explaining how to configure and use the beforeLogin hook for scenarios such as Just-In-Time user provisioning and claim validation. The Auth overview documentation is also updated to reference Auth Hooks and provide a brief explanation.

New Auth Hooks documentation:

• Added a comprehensive guide (docs/guides/auth/hook.md) detailing the concept, configuration, execution flow, handler arguments, error handling, and example use cases for Auth Hooks, specifically the beforeLogin hook.

Auth overview enhancements:

• Added "Auth Hooks" to the feature list in the Auth overview and linked to the new guide.
• Introduced a new section summarizing Auth Hooks and their purpose, with a link to the detailed guide.