Migration guide to v3

**Is your feature request related to a problem? Please describe.**
A simple upgrade exposes several issues (like 86 pycharm warnings as it can no longer determine constructor arguments for Output classes).

**Describe the solution you'd like**
Release notes are nice, but it's details. I'd like to have a broad overview of the major changes and issues you'd face upgrading to this new major version. What are the new core concepts? What are the backwards incompatible changes? Which mainstream dependant packages are not yet ready? Which design patterns are still supported, but deprecated and should we move away from?

**Describe alternatives you've considered**
Read the changelog, but things like `Rename op_name to operation_name` lack all context and don't provide a high level view.