Move package organization information into public repo documentation

The large application codebases are not laid out in an obvious manner, unless you've already worked on a project within the evergreen-ci org already. It would be nice to include a high-level ARCHITECTURE.md file. Alternatively, it would be nice to instead put package-level comments in the packages within the repos.

Although we have internal Google docs describing the layout of projects, it might be preferable to instead have public docs (i.e. within the repos) to make it easier for new contributors to understand how the code is laid out (e.g. where to find particular modules and features). By connecting the file directly to the repo itself, it also makes it more obvious where someone should get started without the pre-existing knowledge of, and access to, the Google docs. I imagine it would be much easier to remember to consult and occasionally update the package high-level overview if it was just a file in the repo instead of in a standalone internal Google doc. The Google doc has a habit of becoming outdated because they're not tied to the repo at all.