docs: document JSON value serialization for Deque

[Hugohong258]

Deque indexing and iteration fail with JSONDisk because queue keys bypass Disk.put() and are later passed to JSONDisk.get(). Add the limitation and the value-only JSON subclass suggested in #291 to the tutorial, with a runnable Deque example. This documents the workaround without changing serialization or queue behavior.

Validation: reproduced the indexing and iteration failures against the current source and executed the exact new example. Nine checks cover the failure, ordinary push/pull, indexing, iteration, both deque ends, reopening the cache, and default key lookup with the subclass. The Sphinx HTML build passes with warnings treated as errors after installing the optional Django dependency required by the existing API documentation. The full runtime/stress suite was not run. git diff --check passes.