[Docs] Added MongoDB persistence in how-tos (#960)

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Co-authored-by: Vadym Barda <vadim.barda@gmail.com>
Co-authored-by: Vadym Barda <vadym@langchain.dev>

docs/mkdocs.yml

@@ -137,6 +137,7 @@ nav:
           - Delete messages: how-tos/memory/delete-messages.ipynb
           - Add summary of the conversation history: how-tos/memory/add-summary-conversation-history.ipynb
           - Create custom checkpointer using Postgres: how-tos/persistence_postgres.ipynb
+          - Create custom checkpointer using MongoDB: how-tos/persistence_mongodb.ipynb
       - Human-in-the-loop:
           - Add breakpoints: how-tos/human_in_the_loop/breakpoints.ipynb
           - Wait for user input: how-tos/human_in_the_loop/wait-user-input.ipynb