Write an Architecture.md

[Miles-Garnsey]

What is missing?

We don't document code structure or what should be in pkg vs controllers (or why pkg even exists vs an internal dir). We also don't document what an envtest is, when to use one vs a e2e test or unit test, what the

Why do we need it?

This is an OSS project and we should enable new contributors with as much info as possible.

┆Issue is synchronized with this Jira Story by Unito
┆Issue Number: K8OP-11