doc: Adds v1.39.0 upgrade guide (#3522)

* v1.39.0 upgrade guide

* Update docs/guides/1.39.0-upgrade-guide.md

Co-authored-by: Leo Antoli <[email protected]>

* pr comment

* pr comments

---------

Co-authored-by: Leo Antoli <[email protected]>

Author: oarbusi

docs/guides/1.39.0-upgrade-guide.md

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+---
+page_title: "Upgrade Guide 1.39.0"
+---
+
+# MongoDB Atlas Provider 1.39.0: Upgrade and Information Guide
+
+The Terraform MongoDB Atlas Provider version 1.39.0 introduces the following new features and fixes:
+
+## New Resources, Data Sources, and Features
+
+- Adds support for the `MONGODB_ATLAS_PUBLIC_API_KEY` and `MONGODB_ATLAS_PRIVATE_API_KEY` environment variables which are widely used across the MongoDB ecosystem. These provide an alternative way to configure authentication alongside the existing `MONGODB_ATLAS_PUBLIC_KEY` and `MONGODB_ATLAS_PRIVATE_KEY` variables.
+- Adds support for the `mongodbatlas_federated_database_instance` resource and related data sources in the Azure cloud provider configuration through the new `azure` attribute. To learn more, see the [resource documentation](https://registry.terraform.io/providers/mongodb/mongodbatlas/latest/docs/resources/federated_database_instance).
+
+## Bug Fixes
+
+- **Organization Import Fix**: Fixes an issue where the `org_id` was not being set correctly when importing an existing organization using the `mongodbatlas_organization` resource.
+
+## Helpful Links
+
+* [Report bugs](https://github.com/mongodb/terraform-provider-mongodbatlas/issues)
+* [Request Features](https://feedback.mongodb.com/forums/924145-atlas?category_id=370723)
+* [Contact Support](https://docs.atlas.mongodb.com/support/) covered by MongoDB Atlas support plans, Developer and above.