Add tutorial section on child FFD

[marcomangano]

Type of issue

• Documentation update

Description

As per title, we are not currently providing documentation and example scripts for the use of child FFDs.
We should add a paragraph at the end of this section or create a new advanced features section in the tutorial

[shamsheersc19]

Will the overset tutorial that @DavidAnderegg is working on include child FFD stuff? @anilyil

[anilyil]

I think he is using VSP for the geometry so probably not.

[DavidAnderegg]

@shamsheersc19 , I am only using VSP.