Update documentation for neoclassical models, configuration, and registry

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Update documentation for neoclassical models, configuration, and registry

• Document the composite analytical neoclassical model and its four
  sub-models (bootstrap_current, conductivity, transport,
  poloidal_velocity), including Redl conductivity, Angioni-Sauter +
  Shaing transport, and Kim poloidal velocity, in physics_models.rst,
  code_structure.rst, and overview.rst.
• Document the neoclassical configuration options (model_name='analytical',
  conductivity: 'redl', transport Shaing parameters, and
  poloidal_velocity sub-config) in configuration.rst.
• Document custom top-level neoclassical model registration
  (torax.neoclassical) in model_integration.rst.