docs(sdk): Add comprehensive README for SP1 SDK

[MozirDmitriy]

Motivation

The SP1 SDK, being the primary interface for developers to build zero-knowledge applications, lacked comprehensive documentation in its directory. A well-structured README is essential for:

• Helping new developers understand the SDK's capabilities
• Providing quick reference for common usage patterns
• Explaining the relationship with other SP1 components
• Making the codebase more maintainable and accessible

Solution

Added a detailed README.md to the SDK crate that includes:

• Clear overview of the SDK's purpose and capabilities
• List of key features supported by the SDK
• Usage instructions with dependency information
• Practical code example showing basic program structure
• Component breakdown of SDK features
• References to related SP1 components
• Links to detailed documentation

The documentation follows the project's existing style and provides essential information for both new and experienced developers.

PR Checklist

• Added Documentation
• Added Tests (N/A - documentation only)
• Breaking changes (No)

[nhtyy]

Documentation seems to be for imagined functionality