Add system architecture and data flow diagrams

[jhodapp]

Suggestion / Feature Request

In order to help document design decisions and current state for those already in the project, and to help new developers looking to get involved, add the following diagrams to the Rust docs:

1. Architectural overview of the crate and how all of the major modules/structs relate to one another
2. One or more flow charts showing important concepts of how data flows from the high level interfaces down to input/output over the SPI bus

Definition of Done

• Diagrams have been added to Rust docs
• Diagrams have been reviewed by Rust Never Sleeps developers less familiar with the crate's inner workings, and have indicated they are helpful in understanding the inner workings