docs: Enhance documentation for adding Uno projects to existing solutions

[Copilot]

Problem

The issue reported that documentation was missing for adding Uno Platform projects to existing .NET solutions. Upon investigation, I found that while documentation did exist, it had several critical problems:

1. Incorrect cross-references: The common issues documentation was pointing developers to the wrong guide (adding platforms to existing Uno projects instead of adding Uno projects to existing solutions)
2. Incomplete guidance: The existing documentation was too brief and lacked practical examples
3. Poor discoverability: The table of contents entry was unclear ("New Projects in Existing Solutions")

This created confusion for developers trying to integrate Uno Platform into their existing .NET solutions - a common scenario for gradual migration or enterprise integration.

Changes Made

Fixed Cross-Reference Error

• Updated common-issues-vs2022.md to correctly reference the "Adding an Uno Platform Project to an existing solution" guide instead of the unrelated "Adding Platforms to an Existing Project" guide

Comprehensive Documentation Enhancement

Completely rewrote how-to-add-uno-to-an-existing-solution.md with:

• Clear introduction explaining use cases (gradual migration, enterprise integration, shared components)
• Prerequisites section with required tools and setup
• Detailed step-by-step guide covering:
  • Creating Uno projects with dotnet CLI and Visual Studio examples
  • Handling global.json file integration
  • Managing NuGet Central Package Management (CPM) scenarios
  • Adding projects to existing solutions
• Comprehensive troubleshooting section for common integration issues
• Related guides and next steps for continued development

Improved Discoverability

• Updated table of contents entry from "New Projects in Existing Solutions" to "Adding Uno Projects to Existing Solutions" for clarity
• Added proper cross-references to related documentation

Infrastructure Improvements

• Updated documentation .gitignore to properly exclude build artifacts

Impact

This enhancement addresses a common developer scenario where teams want to:

• Add cross-platform capabilities to existing .NET solutions
• Gradually migrate from other UI frameworks to Uno Platform
• Integrate Uno Platform into established enterprise workflows

The documentation now provides clear, actionable guidance with practical examples and troubleshooting help, making it much easier for developers to successfully integrate Uno Platform into their existing solutions.

All changes have been validated with successful documentation builds and proper cross-reference resolution.

Fixes #20780

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[unodevops]

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