docs: add type-mapping table + document FieldUpdate/bulk-write

[zeevdr]

Description

The Python SDK's core value is typed coercion, but docs only list five scalar types and never document the full Go-to-Python mapping or the FieldUpdate bulk-write shape; several public kwargs and methods are undocumented.

Acceptance criteria

• Add a Go-type -> Python-type table: time -> datetime, duration -> timedelta, url -> str (opendecree.URL), json -> dict/list; add datetime/dict/list to the supported get() types.
• Document the interceptors constructor kwarg and the version-compatibility surface (get_server_version, check_compatibility, IncompatibleServerError).

Source: documentation review, 2026-06-07.