docs(sphinx): add _AssistantNamespaceProxy to nitpick_ignore

## Purpose

Sphinx autodoc renders return types as cross-references in HTML.
`Pinecone.assistant` returns `_AssistantNamespaceProxy`, a private
class, which would produce a broken reference link or build warning
in nitpicky mode without a `nitpick_ignore` entry.

## Solution

Added `pinecone.client._assistant_namespace_proxy._AssistantNamespaceProxy`
to the `nitpick_ignore` list in `docs/conf.py`, following the same
pattern already used for `PineconeConfig`, `RetryConfig`, `HTTPClient`,
and `AsyncHTTPClient`.

Author: jhamon

docs/conf.py

@@ -72,6 +72,7 @@
     ("py:class", "pinecone._internal.config.RetryConfig"),
     ("py:class", "pinecone._internal.http_client.HTTPClient"),
     ("py:class", "pinecone._internal.http_client.AsyncHTTPClient"),
+    ("py:class", "pinecone.client._assistant_namespace_proxy._AssistantNamespaceProxy"),
 ]
 
 myst_enable_extensions = ["colon_fence", "deflist"]