feat: add MongoDB Atlas webhook provider

[selenaalpha77-sketch]

Closes #1839

Summary

Adds a MongoDB Atlas provider that receives alert webhooks from Atlas monitoring. Follows Keep's webhook provider pattern (same as Checkmk, SNMP, Wazuh).

Files added:

• keep/providers/mongodbatlas_provider/mongodbatlas_provider.py — webhook provider, _format_alert maps Atlas payload to AlertDto
• keep/providers/mongodbatlas_provider/alerts_mock.py — 3 sample payloads (OPEN, CLOSED, ACKNOWLEDGED)
• keep/providers/mongodbatlas_provider/__init__.py
• docs/providers/documentation/mongodbatlas-provider.mdx — setup guide with payload reference

Status mapping: OPEN/TRACKING → FIRING, CLOSED/CANCELLED → RESOLVED, ACKNOWLEDGED → ACKNOWLEDGED

Severity mapping: inferred from typeName + eventTypeName keywords (e.g. NO_PRIMARY → CRITICAL, HOST_DOWN → HIGH)

No external dependencies required.

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