Expand architecture documentation

Add an [ARCHITECTURE.md file](https://matklad.github.io/2021/02/06/ARCHITECTURE.md.html) to allow new contributors to learn about the physical architecture of the project easily.

Create ARCHITECTURE.md in the repository. Start by reviewing the project structure and identifying the physical components a new contributor needs to understand. Done means the document gives newcomers a clear overview of the project's architecture and how its parts fit together.

Written by the indexing model from the issue text.