[Documentation] No description of Map in-memory layout

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](https://issues.apache.org/jira/browse/ARROW-14131) / @jorgecarleitao

_{**Note**: *This issue was originally created as [ARROW-14131](https://issues.apache.org/jira/browse/ARROW-14131). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Start with the physical memory layout page at https://arrow.apache.org/docs/format/Columnar.html#physical-memory-layout and inspect the Map definition in schema.fbs. Document Map's in-memory layout and when it should be used, so the page covers this schema type and is sufficient to implement and understand it.

Written by the indexing model from the issue text.