Refactor GP kernel parameters from amplitude to standard deviation

- Updated kernel parameter names from `log_gp_amp` to `log_gp_sigma` across the codebase to reflect the change in terminology.
- Adjusted documentation to clarify that the first parameter for all kernels is now the standard deviation (sigma) instead of amplitude.
- Modified configuration examples and tests to use the new parameter naming convention.
- Ensured consistency in the handling of spectral mixture components and their associated parameters.