MongoDB as Chat Memory Provider

[Pash10g]

This pull request includes documentation updates for the MongoDB Chat Memory node in n8n. The changes introduce a new documentation page for the MongoDB Chat Memory node and update the credentials page to include this new node.

Documentation updates:

• docs/integrations/builtin/cluster-nodes/sub-nodes/n8n-nodes-langchain.memorymongochat.md: Created a new documentation page for the MongoDB Chat Memory node, detailing its usage, parameters, and related resources.
• docs/integrations/builtin/credentials/mongodb.md: Added the MongoDB Chat Memory node to the list of nodes that can use MongoDB credentials for authentication.

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