docs: Mkdocs Documentation #15

[CoolatMax]

This Pull Request addresses Issue #15 regarding documentation improvements. It transforms the documentation from a single, static README into a professional, multi-page MkDocs site with a clear Information Architecture.

Key Changes

• Structural Reorganization: Decomposed the README.md into a structured docs/ directory featuring dedicated sections for Explanation, Guides, and Reference.

• Enhanced Navigation: Updated mkdocs.yml with a logical navigation hierarchy to improve discoverability of core features.

• Expanded Getting Started Guide: Revised the tutorial to include environment verification and a practical example of compiling a vectorized SIR model.

• Technical Deep Dives: Added comprehensive documentation for RHS Specifications (kind: expr and kind: transitions), spatial Axes, and Mixing Kernels.

• API Documentation: Integrated a structured reference for primary functions like compile_spec and normalize_rhs.

Verification

• The site was verified locally using mkdocs serve.

• All navigation links and code snippets were checked for accuracy against the source logic in src/op_system.

Closes: ISSUE #15