Onboard MongoDB Flex user (#88)

* Onboard user

* Add examples and generate docs

* Adjustments after review

examples/resources/stackit_mongodbflex_instance/resource.tf

@@ -0,0 +1,18 @@
+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"
+  }
+}