Add comprehensive contributor guide

- Pedagogical philosophy: theory before code, Lax equivalence foundation,
  error analysis throughout, progressive complexity, reproducibility
- Document content gaps from roadmap analysis: elliptic PDEs, high-order
  schemes, SBP/mimetic methods, curvilinear grids, adjoint methods
- Contribution guidance for each gap with SymPy/Devito examples
- Chapter template for new content structure
- Standards for SymPy math derivations, Devito solver patterns
- Code verification hierarchy (exact solutions, MMS, conservation)
- Plotting, image generation, and Quarto guidelines
- Testing, linting, and local development instructions
- Attribution and licensing requirements

Author: ggorman