Add comprehensive onboarding guides for junior Rust engineers

[Copilot]

This PR adds two comprehensive onboarding guides specifically designed to help junior Rust engineers get productive quickly on the Alys Bitcoin sidechain project.

What's Added

1. Quick Start Guide (QUICK_START_GUIDE.md)

A focused 15-minute introduction that gets new engineers up and running immediately:

• Fast setup with essential prerequisites only
• Project structure overview with key files highlighted
• Real code examples from the actual codebase (AuxPoW module)
• Common command reference for daily development
• Immediate troubleshooting for the most frequent issues

2. Comprehensive Guide (JUNIOR_RUST_ENGINEER_GUIDE.md)

An exhaustive 12-section deep-dive covering everything a junior engineer needs:

• Complete project understanding - Bitcoin sidechains, merged mining, federation consensus
• Detailed setup instructions with real troubleshooting for network/certificate issues
• Codebase architecture - deep dive into app/, crates/, and contracts/ structure
• Rust patterns in practice - error handling, async programming, state management as used in Alys
• Development workflows - building, testing, debugging with practical examples
• Contributing guidelines - code style, PR process, security considerations

Key Features

Practical and Tested: All instructions have been validated against the actual codebase, including common build failures and their solutions.

Real Examples: Instead of abstract code samples, the guides use actual code from the project:

#[test]
fn empty_merkle_branch() {
    let hash_aux_block = BlockHash::from_byte_array([1; 32]);
    let blockchain_branch = MerkleBranch {
        branch_hash: vec![],
        branch_side_mask: 0,
    };
    // Real test from app/src/auxpow.rs showing merkle proof validation
}

Comprehensive Troubleshooting: Addresses real issues encountered during development:

• Network timeouts during dependency resolution
• SSL certificate verification failures
• Lighthouse dependency build issues
• Common runtime and testing problems

Progressive Learning Path: Guides engineers from basic setup through advanced contribution patterns, with clear next steps at each level.

Target Audience

Junior Rust engineers who are:

• New to blockchain development
• Unfamiliar with Bitcoin sidechains and merged mining concepts
• Looking to contribute to a complex, production-grade Rust codebase
• Need practical guidance beyond basic Rust tutorials

The guides complement the existing technical documentation by focusing specifically on the developer experience and practical engineering concerns.

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