doc(facade/PC): add README

[skaldarnar]

Contains

Add a README for the PC Facade explaining basic usage and architecture.

Based on the discussion on Document Execution Flows in the Engine on Discord.

How to test

n/a

Give it a good read and let me know about anything that sounds off or leaves open questions.

Outstanding before merging

n/a

[skaldarnar]

@jdrueckert thanks for taking care of implementing the suggestions and getting this merged 💚