Update documentation for GaNDLF Task Runner

[psfoley]

This task involves creating specific documentation for GaNDLF for both:

1. Simulation (single node experimentation)
2. Multi node federations

The GaNDLFTaskRunner uses the Aggregator Based Workflow, so this documentation should be based on https://openfl.readthedocs.io/en/latest/running_the_federation.html#aggregator-based-workflow