Update Next.js auth docs examples (#86361)
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### What?
Updated the `Layouts and auth checks` section in the authentication
documentation (`docs/01-app/02-guides/authentication.mdx`).
### Why?
The previous example demonstrating authentication checks within layouts
was confusing and contradicted the documentation's own advice against
this pattern. This update aims to provide clearer, recommended
approaches for implementing authentication checks.
### How?
- Removed the misleading layout-based authentication example.
- Introduced a new subsection with an example showing authentication
checks in a page component.
- Added another new subsection with an example demonstrating
authentication checks in a leaf component.
- Enhanced the "Good to know" section with a reminder about the
necessity of authorization checks for Server Actions.
Closes NEXT-
Fixes #
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