docs: Add comprehensive OAuth2 authentication documentation

[devin-ai-integration]

OAuth2 Documentation Improvements

This PR adds comprehensive OAuth2 authentication documentation to improve the user experience for developers implementing OAuth2 in their Appsmith applications.

Changes

• Add centralized OAuth2 authentication guide with best practices
• Include OAuth2 flow diagrams and troubleshooting information
• Update navigation structure to include OAuth2 documentation section
• Add detailed error handling and provider-specific guides

Testing

• Documentation files have been created and properly linked
• Sidebar navigation has been updated and tested
• Markdown formatting has been verified
• All internal documentation links are valid

Link to Devin run: https://app.devin.ai/sessions/984e3d6001194e0a9483245c75161a77

[devin-ai-integration]

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Name	Status	Preview	Comments	Updated (UTC)
appsmith-docs	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Jan 8, 2025 10:24am

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