add cosmos db sample for checkpointer

[TheovanKraay]

Overview

added cosmos db sample for checkpointer in short term memory section

Type of change

Update existing documentation

Related issues/PRs

N/A

Checklist

• I have read the contributing guidelines
• I have tested my changes locally using docs dev
• All code examples have been tested and work correctly
• I have used root relative paths for internal links
• I have updated navigation in src/docs.json if needed
• I have gotten approval from the relevant reviewers
• (Internal team members only / optional) I have created a preview deployment using the Create Preview Branch workflow

Additional notes

There are actually many checkpointer store implementations now: https://pypi.org/search/?q=langgraph-checkpoint. Perhaps this doc could further be revised to make that clear. However, I cannot comment authoritatively on those (I am PM in Cosmos DB engineering team). so just adding the Cosmos DB sample in addition to the existing Postgres sample.

[Copilot]

Pull Request Overview

This PR adds documentation for using Azure Cosmos DB as a checkpoint saver for LangChain agents, providing an alternative to the existing PostgreSQL example.

Key changes:

• Added installation instructions and Python code example for the langgraph-checkpoint-cosmosdb package
• Included configuration details for authentication and database/container setup
• Added a note about sync/async support and RBAC credentials

[TheovanKraay]

Note: the earlier code snippet commit suggestion with the context manager pattern (with CosmosDBSaver(...) as checkpointer) will not work because the CosmosDBSaver class does not implement __enter__ and __exit__ methods on its main __init__() constructor - only the separate from_conn_info() class method returns a context manager.

The standard usage pattern is direct instantiation (checkpointer = CosmosDBSaver(database_name="...", container_name="...")) followed by passing it to the graph's compile method.