[Additional feature] UML for PMR IV

[joeyouss]

While working with PMR IV and trying to understand how different and so many functions work together, I could hand draw some diagrams for relationship between elements. I propose to convert it into a UML which may be used by contributors in the future as a reference guide and diagram rather than visiting each function and readings docstrings while losing track of the program control.

Tool to be used: Lucid Chart

[PulkitMishra]

Low priority. feel free to send a PR and with the same