add documentation to the kgx merge doc that shows when to use cat-merge as well as a set of steps to use merge config

[sierra-moxon]

simple documentation on what KGX merge does as well would be incredibly useful.
The use case this ticket originated from was from Brook S. merging phenio with her graph and seeing some strange merge results.

• merge config doc needs an example and a link to the actual merge config spec.
• need a step by step guide of how to do a merge
• need a bare minimum merge_config.yaml file (https://github.com/Knowledge-Graph-Hub/kg-template/blob/master/merge.yaml)
• why we use cat-merge (speed, transparent mappings, more mappings needed after the KGX)