Update README to accurately reflect current codebase implementation

[Copilot]

This PR addresses significant discrepancies between the README documentation and the actual codebase implementation. The README contained outdated information and references to features that are not yet implemented.

Key Changes Made

Version and Package Information

• Updated package version from 1.0.0 to the actual current version 0.0.3 in installation examples
• Clarified .NET 9.0 SDK requirement to match the project's net9.0 target framework

Removed References to Non-Existent Features

• CLI Tool: Removed all references to MarkItDown.Cli project which doesn't exist in the current codebase
• Azure DocumentIntelligence: Removed usage examples showing implemented Azure integration. While DocumentIntelligenceOptions exists, the actual converter implementation is not yet available

Updated Project Structure

• Fixed project structure documentation to reflect the actual directory layout without the non-existent CLI project
• Updated the folder structure diagram to show only existing components

Build and Development Commands

• Updated build commands to explicitly reference project files since .slnx format requires .NET 9 SDK
• Changed from generic dotnet build to dotnet build src/MarkItDown/MarkItDown.csproj
• Updated test commands to use specific project paths

Roadmap Clarification

• Updated the roadmap section to clearly distinguish between implemented features and planned features
• Moved CLI tool and Azure DocumentIntelligence to the "planned" category with appropriate context

Verified Accurate Information

All remaining documentation has been validated against the actual codebase:

• Format support table matches the converters registered in MarkItDown.cs
• All API examples use correct method signatures and parameters
• Code examples reference properly defined constants like ConverterPriority.SpecificFileFormat
• Package dependencies listed match those in the .csproj file

The README now provides accurate, actionable information for developers wanting to use or contribute to the project, with working build instructions and correct usage examples.

Fixes #[issue-number]

Fixes #13

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