[Phase 5] Create comprehensive documentation

[marioarce]

Task

Create comprehensive documentation for the Sitecore thread-safe context package.

Deliverables

• Create docs/PowerCSharp.Sitecore.md:
  • Installation instructions for each target framework
  • Web.config registration example (.NET Framework)
  • Middleware registration example (.NET 6/8)
  • Runtime dependency documentation (Sitecore.Kernel must be available)
  • Version compatibility matrix
  • Migration guide from Sitecore.Context to ThreadSafeContext
  • Troubleshooting common issues
• Update main README.md with Sitecore package section
• Add XML documentation comments to all public APIs
• Create code samples for common scenarios

Acceptance Criteria

• Documentation is complete and accurate
• Examples work for both .NET Framework and .NET 6/8
• Runtime dependencies clearly documented
• Migration guide is easy to follow
• XML comments on all public APIs

[marioarce]

I've decided not to pursue this feature within this repository.
Instead, we are implementing it as part of our other project, Axlis, where it aligns better with the architectural goals.

I am closing this issue/milestone here to keep our tracker clean.