docs(ui): add Error List documentation (#111)

Add comprehensive documentation for working with the Error List:
- ErrorListProvider usage
- ErrorTask properties reference
- Error categories (Error, Warning, Message)
- Task categories (BuildCompile, CodeSense, etc.)
- Navigation support for clicking to source
- Adding error codes
- Project-specific errors
- Complete validation service example
- Best practices

Closes #44

Author: CalvinAllen