feat: warn when experimental features are used (#669)

##### SUMMARY

- Document how users can learn about experimental features.
- Document how developers can release experimental features.
- Log a warning when experimental features are being used.

##### ISSUE TYPE

- Feature Pull Request

README.md

@@ -75,3 +75,33 @@ ansible-test integration --color --local -vvv hetzner.hcloud.server // Executed
 ## Releasing a new version
 
 If there are releasable changes, `release-please` will open a PR on GitHub with the proposed version. When this PR is merged, `release-please` will tag the release.
+
+## Releasing experimental features
+
+To publish experimental features as part of regular releases:
+
+- an announcement, including a link to a changelog entry, must be added to the release notes.
+
+- an `Experimental` notice, including a link to a changelog entry, must be added to the experimental plugins documentation:
+
+  ```py
+  DOCUMENTATION = """
+  ---
+  module: product
+
+  description:
+      - Create, update and manage Product on the Hetzner Cloud.
+      - B(Experimental:) Product is experimental, breaking changes may occur within minor releases. See https://docs.hetzner.cloud/changelog#new-product for more details.
+  """
+  ```
+
+- a `Experimental` warning, including a link to a changelog entry, must be logged when experimental plugins are being used:
+
+  ```py
+  product_experimental_warning = experimental_warning_function("Product", "https://docs.hetzner.cloud/changelog#new-product")
+
+  class AnsibleProduct(AnsibleHCloud):
+      def __init__(self, module: AnsibleModule):
+          product_experimental_warning(module)
+          super().__init__(module)
+  ```

plugins/module_utils/experimental.py

@@ -0,0 +1,31 @@
+# Copyright: (c) 2025, Hetzner Cloud GmbH <info@hetzner-cloud.de>
+
+from __future__ import annotations
+
+from .hcloud import AnsibleModule
+
+
+def experimental_warning_function(product: str, url: str):
+    """
+    Create a reusable experimental warning function.
+
+    Usage:
+
+        product_experimental_warning = experimental_warning_function(
+            "Product", "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 experimental product.
+    :param url: Changelog URL announcing the experimental product.
+    """
+    message = f"Experimental: {product} is experimental, breaking changes may occur within minor releases. See {url} for more details."
+
+    def fn(module: AnsibleModule):
+        module.warn(message)
+
+    return fn