Add contributor documentation

This should include a `CONTRIBUTING.md` file, plus an `ARCHITECTURE.md` describing the high-level architecture of the project. In addition, each source file needs doc and implementation comments.

Start by reviewing the repository structure and existing documentation, then determine what belongs in CONTRIBUTING.md and ARCHITECTURE.md. The issue also requests documentation and implementation comments in every source file, but does not define the expected scope or content. Done means both named documents exist and all source files contain the requested comments.

Written by the indexing model from the issue text.