docs: document parameters in conversion.py (closes #611)

[christianescamilla15-cell]

Summary

Addresses #611 by adding parameter descriptions and math formulae to several functions in bilby/gw/conversion.py. Used Table E1 and Appendix E of Romero-Shaw et al. 2020 (arXiv:2006.00714) as the reference for spin parameter definitions, and Peters & Mathews 1963 for the chirp mass formula.

Functions updated

generate_component_spins

Full documentation of all 11 input spin parameters (`theta_jn`, `phi_jl`, `tilt_1/2`, `phi_12`, `a_1/2`, masses, `reference_frequency`, `phase`) with units, ranges, and physical meaning. Documents both the precessing conversion path and the aligned-spin fallback (`chi_1`, `chi_2`). Lists all 9 output keys with descriptions.

Mass conversion functions

• `component_masses_to_chirp_mass`: Added the formula $\mathcal{M} = (m_1 m_2)^{3/5} / (m_1 + m_2)^{1/5}$ and reference to Peters & Mathews 1963.
• `component_masses_to_symmetric_mass_ratio`: Added $\eta = m_1 m_2 / (m_1 + m_2)^2$ with the range $\eta \in (0, 1/4]$.
• `component_masses_to_mass_ratio`: Fixed a copy-paste bug where the docstring incorrectly said "Convert the component masses of a binary to its chirp mass". Added $q = m_2/m_1$ and clarified bilby's $q \leq 1$ convention.
• `symmetric_mass_ratio_to_mass_ratio`: Added the inversion formula and explained why the smaller root is chosen.
• `chirp_mass_and_total_mass_to_symmetric_mass_ratio`: Added derivation from $\mathcal{M} = M \eta^{3/5}$.

Also switched to raw strings (`r"""`) so LaTeX in the docstrings renders correctly in the sphinx-generated docs.

Test plan

• No code behavior changed — only docstrings
• Verified `py_compile` passes
• CHANGELOG updated under `[Unreleased]`
• Parameter definitions checked against Table E1 of arXiv:2006.00714

References

• Romero-Shaw, I. et al. "Bayesian inference for compact binary coalescences with bilby: Validation and application to the first LIGO-Virgo gravitational-wave transient catalogue", MNRAS 499, 3295 (2020). arXiv:2006.00714
• Peters, P. C. and Mathews, J. "Gravitational Radiation from Point Masses in a Keplerian Orbit", Phys. Rev. 131, 435 (1963)