[docs]: DIAGRAM - Implement "Black Boxing of Operator Logic" in "Diagram Canvas"

We need a way to "black box" operator logic.

Followigthngs need to be considered:
- Black-Boxing should follow strict rules. There has to be a way to systematically hide internal logic starting from the most detailed level up to the most simple one. The other direction is not needed (and possible)
- We need to introduce levels of black-boxing. IMHO this is directly related to the level of nested operators.

Credits for this idea goes to @kyleryanbanks

Related to #18

Start by reading the Diagram Canvas context and related issue #18, then determine where operator nesting and visibility are represented. Define the black-boxing levels and strict one-way behavior before implementation. Done means the canvas can hide internal operator logic progressively from detailed to simpler levels.

Written by the indexing model from the issue text.