doc: Adds 1.38.0 upgrade guide with new features and bug fixes (#3489)

* docs: Adds 1.38.0 upgrade guide with new features and bug fixes

* Update docs/guides/1.38.0-upgrade-guide.md

Co-authored-by: Copilot <[email protected]>

* address PR comment

* Update docs/guides/1.38.0-upgrade-guide.md

Co-authored-by: davidhou17 <[email protected]>

* Update docs/guides/1.38.0-upgrade-guide.md

Co-authored-by: davidhou17 <[email protected]>

* doc: Address PR comments

* Update docs/guides/1.38.0-upgrade-guide.md

Co-authored-by: davidhou17 <[email protected]>

---------

Co-authored-by: Copilot <[email protected]>
Co-authored-by: davidhou17 <[email protected]>

Author: 3 people

docs/guides/1.38.0-upgrade-guide.md

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+---
+page_title: "Upgrade Guide 1.38.0"
+---
+
+# MongoDB Atlas Provider 1.38.0: Upgrade and Information Guide
+
+The Terraform MongoDB Atlas Provider version 1.38.0 introduces the following new features and fixes:
+
+## New Resources, Data Sources, and Features
+
+- You can now use `mongodbatlas_api_key_project_assignment` to assign an API key to a project. See the [Migration Guide: Project API Key to API Key + Project Assignment](project-api-key-migration) and the [resource documentation](https://registry.terraform.io/providers/mongodb/mongodbatlas/latest/docs/resources/api_key_project_assignment) to learn more.
+- The `mongodbatlas_organization` resource now supports importing an existing organization. See the [import guide](importing-organization) to learn more.
+- The `mongodbatlas_third_party_integration` resource now supports the `send_user_provided_resource_tags` attribute to support sending `$querystats` to Datadog. See the [resource documentation](https://registry.terraform.io/providers/mongodb/mongodbatlas/latest/docs/resources/third_party_integration#DATADOG-1) to learn more.
+
+## Bug Fixes
+
+- Fixes an issue with pagination for plural data sources where only a single item was returned by default when `items_per_page` was undefined.
+
+
+The 1.38.0 release also includes general improvements, bug fixes and several key documentation updates. See the [CHANGELOG](https://github.com/mongodb/terraform-provider-mongodbatlas/blob/master/CHANGELOG.md) for more specific information.
+
+## 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.