docs: Fill documentation book

[dhedey]

• Complete table of contents (SUMMARY.md) with organized sections
• Introduction with motivation, key features, and comparison table
• Getting Started guide with installation, first macro tutorial, and quick reference
• Core Concepts: Streams, Variables, Expressions, Control Flow, and Parsing
• Expression Values overview with detailed type system explanation
• Guides section with error handling documentation
• Stub pages for remaining values, guides, and reference sections

The documentation provides a solid foundation covering:

• Stream literals (%[...]) and raw streams (%raw[...])
• Expression blocks (#{...}) and inline expressions (#(...))
• Control flow (if/for/while/loop) with break/continue
• Parsing with @ patterns
• Error handling with attempt blocks
• Variable destructuring and scoping
• Type system and conversions

Stub files are in place for future expansion of:

• Individual value type pages (integers, floats, etc.)
• Advanced guides (spans, debugging, advanced parsing)
• Reference pages (methods, operators, comparison)