Document rupture parameterization

We don't currently have good documentation and illustration of how fault based ruptures (traditional and fault-system) are initialized and then GMM parameters derived therefrom. Fault, grid and other source rupture implementation details could be added under Implementation Details.

added Docs label

• Check Moschetti et al. (2015) Earthquake Spectra CONUS SSC
• Search for MultiFaultRuptures ai graphics

Also want to document grid and zone implementations with details such as:

• Use of grid optimization tables is triggered by the presence of a non-null opt-distance-bin value in grid-config.json. Additional resampling and smoothing can be applied only if optimization is also enabled. This applies to the incremental MFDs that with fault-system grids as well.