Rework module function documentation

[fhammerschmidt]

Functors is a confusing and overloaded term, we should get rid of it.

• Remove mentions of "Functor" in favor of "Module Function"
• Add a new advanced chapter called "Module Function" with some better examples than the one on the module page.
  • reference the advanced chapter in the module chapter
  • add a note that Module Functions are called Functors in other languages, and were called that as well in ReScript previously
• Add a small example to the syntax lookup where Functors are still missing. Call it (Module: ModuleType) => {} or similar.
• Add module type and module type of to the syntax lookup (see playground link)