Document when to use forward declarations versus #includes

[blipper]

Is your feature request related to a problem? Please describe. Not clear to me what the rules are?

Describe the solution you'd like Convert all forward declarations to include style

Describe alternatives you've considered Clarify the policy and criteria

Additional context Add any other context or screenshots about the feature request here.

[MMory]

We need to document the policy for this. The implicit policy is: only include when needed, don't if forward declarations suffice.

[fabianbs96]

See https://github.com/secure-software-engineering/phasar/wiki/Coding-Conventions#include-vs-forward-declaration