Create context documentation for fz API

[yannrichet]

Created context/ directory with 10 markdown files (4369 lines total) designed for LLM consumption to help understand and suggest fz usage:

• overview.md: Framework introduction and key concepts
• syntax-guide.md: Variable substitution and formula syntax
• core-functions.md: API reference for fzi, fzc, fzo, fzr
• model-definition.md: Model configuration guide
• calculators.md: Execution backend types (sh, ssh, cache)
• formulas-and-interpreters.md: Python and R formula evaluation
• parallel-and-caching.md: Parallel execution and caching strategies
• quick-examples.md: Common patterns and use case examples
• README.md: Documentation guide and usage instructions
• INDEX.md: Quick reference index for finding topics

Each file includes:

• Clear syntax examples with code snippets
• Complete working examples
• Common patterns and best practices
• Cross-references to related topics

This documentation will help LLMs provide better assistance with fz by understanding its syntax, features, and typical usage patterns.