add authentication docs page

[StephDietz]

I've updated the Next.js documentation to include a new page covering the best practices for implementing authentication. This doc covers the essential concepts of authentication in Next.js, focusing on practical strategies, effective session management, and robust authorization techniques.

Key aspects of the update:

• Core Authentication Concepts: Introduced foundational concepts of authentication, detailing their relevance and application in Next.js projects.
• Diverse Authentication Strategies: Expanded on various authentication strategies, providing insights on when and how to use them effectively in different scenarios.
• Implementing Authentication in Next.js: Guided through the implementation process, illustrating the use of middleware, server actions, and route handlers to create a secure and user-friendly authentication system.
• Session Management Techniques: Discussed different approaches to session management, highlighting their pros and cons to aid in choosing the most suitable method for specific project needs.
• Securing Application Access: Detailed how to protect routes and manage user permissions, ensuring that access is granted appropriately based on user roles and authentication status.
• Next.js Compatible Solutions: Included references to several compatible authentication solutions for Next.js, facilitating easy integration and setup.

[ijjk]

Allow CI Workflow Run

• approve CI run for commit: 5301848

Note: this should only be enabled once the PR is ready to go and can only be enabled by a maintainer

Addressed

[sokra]

This causes a build error on next-site:

next-site:dev-webpack: ✘ [ERROR] Unexpected character `!` (U+0021) before name, expected a character that can start a name, such as a letter, `$`, or `_` (note: to create a comment in MDX, use `{/* text */}`) [plugin @mdx-js/esbuild]
next-site:dev-webpack: 
next-site:dev-webpack:     _mdx_bundler_entry_point-abe20b89-718a-4640-8893-fcb291d00e45.mdx:541:4:
next-site:dev-webpack:       541 │    <!-- TODO: Change link to authjs.dev when new documentation is...
next-site:dev-webpack:           ╵     ^
next-site:dev-webpack: 
next-site:dev-webpack: Error: Found 1 problems in 600 documents.
next-site:dev-webpack: 
next-site:dev-webpack:  └── Encountered unexpected errors while processing of 1 documents. This is possibly a bug in Contentlayer. Please open an issue.
next-site:dev-webpack:      
next-site:dev-webpack:      • "docs/02-app/01-building-your-application/09-authentication/index.mdx": UnexpectedMDXError: Error: Build failed with 1 error:
next-site:dev-webpack:      _mdx_bundler_entry_point-abe20b89-718a-4640-8893-fcb291d00e45.mdx:541:4: ERROR: [plugin: @mdx-js/esbuild] Unexpected character `!` (U+0021) before name, expected a character that can start a name, such as a letter, `$`, or `_` (note: to create a comment in MDX, use `{/* text */}`)
next-site:dev-webpack:      

[sokra]

fix here: #60824

[StephDietz]

fix here: #60824

We have the fix queued here