Adds documentation and flowchart for update script refactor (#1076)

* first pass at update-bcd data flow documentation

* updates data-flow docs

* adds flowchart diagram

* updates json markdown formatting

* Apply suggestions from code review; format with Prettier

* Convert flowchart to Mermaid

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Co-authored-by: Queen Vinyl Da.i'gyu-Kazotetsu <[email protected]>

Author: ChrisC