Arrow: `Foldable` docs

[raulraja]

[richard-gibson]

Happy to take this if nobody is working on it

[pakoito]

@richard-gibson Perfect! Thank you :D You can check the format we use in other typeclass docs:

• We have a short intro mentioning what behavior they abstract, and which other typeclasses it inherits from
• sometimes a section depicting the main use case is placed here, i.e. Applicative has a long desctiption of applicative builders
• then go through the main combinators/operators/functions with a short description of what they can be used for. This also includes a snippet
• afterwards we mention any piece of syntax available for the operators
• then we have a short section mentioning that Arrow provides laws for this typeclass
• and finally we link all the datatypes that provide an implementation of this typeclass

[AdrianRaFo]

I'll take this if there is no problem