Add documentation for MAP data type

[ray6080]

MAP is supported but missed in doc's data type page.

[prrao87]

@ray6080 Could you point me to the tests for the map function? I wasn't able to locate it in the source. Or, maybe someone else could post example snippets that we could add to the data types page in the docs. Let me know either way.

[ray6080]

Sorry missed the comment here 😅. I think @acquamarin is the best to provide examples and draft doc, as he implemented this data type, so you can review and edit further.

[acquamarin]

I think we do have doc for MAP datatype but somehow it doesn't show up on our doc website:
https://github.com/kuzudb/kuzu-docs/blob/main/src/content/docs/cypher/data-types/map.md