[Documentation] No description of Map in-memory layout

[asfimport]

The page https://arrow.apache.org/docs/format/Columnar.html#physical-memory-layout has no description of the Map type, even though it is part of the schema.fbs. This makes it difficult to implement and understand when it should be used.

Reporter: Jorge Leitão / @jorgecarleitao

_{Note: This issue was originally created as ARROW-14131. Please see the migration documentation for further details.}