[TASK] Document our approach to assertions and type checks

[oliverklee]

Fixes #1330

[lukaszuznanski]

Should this way for decision for ADR/Spikes and then document it this way or documentation is just fine?

[oliverklee]

In our last call, we decided this:

Oli already has experience with that and will add a PR to EXT:tea documentation to document the best practices, so we all can learn and follow the advice.

So we can use this as-is without requiring an additional decision.