Add an ARCHITECTURE.md to the repo

Currently it is difficult to get a "big picture" understanding of Bevy's code base. Adding an ARCHITECTURE.md - described by Maklad [here](https://matklad.github.io/2021/02/06/ARCHITECTURE.md.html) - would be a good first step toward providing mental scaffolding.

This was previously brought up in the [CONTRIBUTING.md issue](https://github.com/bevyengine/bevy/issues/910#issuecomment-814520446).

Start with the repository structure, CONTRIBUTING.md, and Maklad's linked ARCHITECTURE.md article. Map Bevy's codebase at a high level and add ARCHITECTURE.md so newcomers have the requested big-picture mental scaffolding; the issue does not name specific files or tests.

Written by the indexing model from the issue text.