Onboard MongoDB Flex user (#88)

* Onboard user

* Add examples and generate docs

* Adjustments after review

docs/resources/mongodbflex_instance.md

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+---
+# generated by https://github.com/hashicorp/terraform-plugin-docs
+page_title: "stackit_mongodbflex_instance Resource - stackit"
+subcategory: ""
+description: |-
+  MongoDB Flex instance resource schema.
+---
+
+# stackit_mongodbflex_instance (Resource)
+
+MongoDB Flex instance resource schema.
+
+## Example Usage
+
+```terraform
+resource "stackit_mongodbflex_instance" "example" {
+  project_id = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+  name       = "example-instance"
+  acl        = ["XXX.XXX.XXX.X/XX", "XX.XXX.XX.X/XX"]
+  flavor = {
+    cpu = 1
+    ram = 8
+  }
+  replicas = 1
+  storage = {
+    class = "class"
+    size  = 10
+  }
+  version = "5.0"
+  options = {
+    type = "Single"
+  }
+}
+```
+
+<!-- schema generated by tfplugindocs -->
+## Schema
+
+### Required
+
+- `acl` (List of String) The Access Control List (ACL) for the MongoDB Flex instance.
+- `flavor` (Attributes) (see [below for nested schema](#nestedatt--flavor))
+- `name` (String) Instance name.
+- `options` (Attributes) (see [below for nested schema](#nestedatt--options))
+- `project_id` (String) STACKIT project ID to which the instance is associated.
+- `replicas` (Number)
+- `storage` (Attributes) (see [below for nested schema](#nestedatt--storage))
+- `version` (String)
+
+### Read-Only
+
+- `backup_schedule` (String)
+- `id` (String) Terraform's internal resource ID. It is structured as "`project_id`,`instance_id`".
+- `instance_id` (String) ID of the MongoDB Flex instance.
+
+<a id="nestedatt--flavor"></a>
+### Nested Schema for `flavor`
+
+Required:
+
+- `cpu` (Number)
+- `ram` (Number)
+
+Read-Only:
+
+- `description` (String)
+- `id` (String)
+
+
+<a id="nestedatt--options"></a>
+### Nested Schema for `options`
+
+Required:
+
+- `type` (String)
+
+
+<a id="nestedatt--storage"></a>
+### Nested Schema for `storage`
+
+Required:
+
+- `class` (String)
+- `size` (Number)

docs/resources/mongodbflex_user.md

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+---
+# generated by https://github.com/hashicorp/terraform-plugin-docs
+page_title: "stackit_mongodbflex_user Resource - stackit"
+subcategory: ""
+description: |-
+  MongoDB Flex user resource schema.
+---
+
+# stackit_mongodbflex_user (Resource)
+
+MongoDB Flex user resource schema.
+
+## Example Usage
+
+```terraform
+resource "stackit_mongodbflex_user" "example" {
+  project_id  = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+  instance_id = "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx"
+  username    = "username"
+  roles       = ["role"]
+  database    = "database"
+}
+```
+
+<!-- schema generated by tfplugindocs -->
+## Schema
+
+### Required
+
+- `database` (String)
+- `instance_id` (String) ID of the MongoDB Flex instance.
+- `project_id` (String) STACKIT project ID to which the instance is associated.
+- `roles` (Set of String) Database access levels for the user.
+
+### Optional
+
+- `username` (String)
+
+### Read-Only
+
+- `host` (String)
+- `id` (String) Terraform's internal resource ID. It is structured as "`project_id`,`instance_id`,`user_id`".
+- `password` (String, Sensitive)
+- `port` (Number)
+- `user_id` (String) User ID.