ZIR-000: Complete documentation for Arrays/Loops, Backs, and Externs

[zeroecco]

Summary

Complete comprehensive documentation for three previously stub documentation files:

• 99_Arrays_and_Loops.md - Arrays and loop constructs
• 99_Backs.md - Back-references for sequential logic
• 99_Externs.md - External function declarations

Changes

• Arrays and Loops: Added documentation covering array syntax, indexing, literals, loop constructs (indexed for, element iteration, reduce expressions), common patterns (shifts, rotations, packing), and practical examples from the Keccak circuit
• Backs: Documented back-reference syntax (@N), how they enable sequential logic, internal implementation details, use cases (state transitions, counters, pipelining), constraints, and common patterns
• Externs: Explained external functions for nondeterministic operations, declaration syntax, usage patterns, integration with ExternHandler, examples from RISC-V and Keccak circuits, and best practices

Documentation Quality

• Follows existing documentation style and structure
• Includes practical code examples from the codebase
• Provides clear explanations suitable for developers learning Zirgen
• Cross-references related documentation sections
• Includes "Key Takeaways" sections for quick reference

Test Plan

• Reviewed all three documentation files for completeness
• Verified code examples match patterns found in the codebase
• Ensured consistent formatting and style with existing docs
• Added proper navigation links between documentation sections

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