Add radiative transfer equations to docs (#171)

[Deshan-5]

📝 Description

Type: 📝 documentation

Closes #171.

Adds the van Noort et al. (2002) short-characteristics equations to the Transport section of the physics docs — the formal solution (eq 14), integration weights, and finite difference approximations (eqs 15, 16).
Verified against stardis/radiation_field/radiation_field_solvers/base.py.

📌 Resources

• van Noort et al. (2002): https://doi.org/10.1086/338949

🚦 Testing

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Documentation-only change.

☑️ Checklist

• I requested two reviewers for this pull request
• I updated the documentation according to my changes
• I built the documentation by applying the build_docs label