Add example JSON and a dedicated section on handling maps to Sub-Document documentation

This is feedback from a customer call.  It pertains to the Sub-Document SDK pages, e.g. this one for Java: https://docs.couchbase.com/java-sdk/current/howtos/subdocument-operations.html.

• It's not totally clear how to interact with JSON maps/objects.  There's a couple of examples at the start but they're very basic, they don't mention "map" at all (so customer was unable to find on search), and they don't go into nested JSON maps much.
• (Technically the JSON RFC does refer to "objects" rather than "maps", but I think map is fairly commonly used too.  We should refer to both to cover all bases I think?)
• This is more my feedback than customer's, but I think a small example JSON sample at the top that all the examples reference would be very helpful.  The reader has to kind of infer the underlying JSON from the examples at present, and that JSON is different for each example.