refactor: mark module_utils modules as private (#782)

##### SUMMARY

All `module_utils` are now marked as **private**. None of the modules
were intended for public use.

Similar to
https://togithub.com/ansible-collections/community.general/issues/11312

plugins/module_utils/_experimental.py

@@ -0,0 +1,44 @@
+# Copyright: (c) 2025, Hetzner Cloud GmbH <info@hetzner-cloud.de>
+
+# Note that this module util is **PRIVATE** to the collection. It can have breaking changes at any time.
+# Do not use this from other collections or standalone plugins/modules!
+
+from __future__ import annotations
+
+from ._base import AnsibleModule
+
+
+def experimental_warning_function(product: str, maturity: str, url: str):
+    """
+    Create a reusable experimental warning function.
+
+    Usage:
+
+        product_experimental_warning = experimental_warning_function(
+            "Product",
+            "in beta",
+            "https://docs.hetzner.cloud/changelog#new-product",
+        )
+
+        class AnsibleProduct(AnsibleHCloud):
+            def __init__(self, module: AnsibleModule):
+                product_experimental_warning(module)
+                super().__init__(module)
+
+    :param product: Name of the product.
+    :param maturity: Maturity of the product.
+    :param url: Changelog URL announcing the product.
+    """
+    message = f"Experimental: {product} is {maturity}, breaking changes may occur within minor releases. See {url} for more details."
+
+    def fn(module: AnsibleModule):
+        module.warn(message)
+
+    return fn
+
+
+storage_box_experimental_warning = experimental_warning_function(
+    "Storage Box support",
+    "experimental",
+    "https://github.com/ansible-collections/hetzner.hcloud/issues/756",
+)