fix(docs/rerankers): pass rerank=True so examples actually rerank

The reranker examples configured a `reranker` block but called
memory.search(...) without rerank=True. Memory.search() defaults rerank=False
(mem0/memory/main.py:1333) and only reranks 'if rerank and self.reranker'
(:1447), so reranking never ran:

- cohere, zero_entropy, llm, sentence_transformer print result['rerank_score'],
  which doesn't exist unless reranking runs -> KeyError on copy-paste.
- optimization (reranker benchmark) and custom-prompts (reranker-prompt test)
  silently measured un-reranked search.

Add the missing rerank=True flag to every search meant to exercise the reranker.

docs/components/rerankers/models/zero_entropy.mdx

@@ -86,7 +86,7 @@ messages = [
 memory.add(messages, user_id="alice")
 
 # Search with reranking
-results = memory.search("What Italian food does the user like?", filters={"user_id": "alice"})
+results = memory.search("What Italian food does the user like?", filters={"user_id": "alice"}, rerank=True)
 
 for result in results['results']:
     print(f"Memory: {result['memory']}")