Update contributing guide to demonstrate typed PRA#run

#329 and related pull requests changed some of the core SPRAS design. We can use the contributing guide to help teach new contributors about this design and typing within SPRAS.

See https://github.com/Reed-CompBio/spras/pull/329#pullrequestreview-3600369221 and https://github.com/Reed-CompBio/spras/pull/329#issuecomment-3678317558.