[docs] How-To for Writing an Operator

[Austin Pond]

Currently, while we have a fair few docs about design and concepts (and the tutorial), we do not have a best-practices/how-to guide for writing an application operator. The tutorial only briefly touches on the operator code generated by the app-sdk, and many apps will need more complex operator design, so we should have a doc that walks through best practices, when to use a watcher vs a reconciler, deciding what version to watch in a multi-version kind, what logic belongs in a watcher/reconciler vs admission, etc.