Subdoc examples

The subdocument-operations.html page is mostly made up of Python code samples, but not entirely - the text (from the sdk-common repo) will be moved to the individual repos, and the non-Python code samples need translating into Python:

"the second and third code snippet in this section are not python: https://docs.couchbase.com/python-sdk/2.5/subdocument-operations.html#retrieving " - https://issues.couchbase.com/browse/DOC-4552