Onboard MongoDB Flex user (#88)

* Onboard user

* Add examples and generate docs

* Adjustments after review

docs/data-sources/mongodbflex_user.md

@@ -0,0 +1,38 @@
+---
+# generated by https://github.com/hashicorp/terraform-plugin-docs
+page_title: "stackit_mongodbflex_user Data Source - stackit"
+subcategory: ""
+description: |-
+  PostgresFlex user data source schema.
+---
+
+# stackit_mongodbflex_user (Data Source)
+
+PostgresFlex user data source schema.
+
+## Example Usage
+
+```terraform
+data "stackit_mongodbflex_instance" "example" {
+  project_id  = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+  instance_id = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+}
+```
+
+<!-- schema generated by tfplugindocs -->
+## Schema
+
+### Required
+
+- `instance_id` (String) ID of the PostgresFlex instance.
+- `project_id` (String) STACKIT project ID to which the instance is associated.
+- `user_id` (String) User ID.
+
+### Read-Only
+
+- `database` (String)
+- `host` (String)
+- `id` (String) Terraform's internal data source. ID. It is structured as "`project_id`,`instance_id`,`user_id`".
+- `port` (Number)
+- `roles` (Set of String)
+- `username` (String)