feat: add hetzner.hcloud.all action group (#396)

##### SUMMARY

This allows to use `module_defaults` against all the modules at once
using the new `hetzner.hcloud.all` `action_group`.

You can now pass the `api_token` argument using module_defaults:

```yaml
- name: Demonstrate the usage of the 'hetzner.hcloud.all' module_defaults group
  hosts: localhost
  connection: local

  module_defaults:
    group/hetzner.hcloud.all:
      api_token: "{{ _vault_hcloud_api_token }}"

  tasks:
    - name: Create a volume
      hetzner.hcloud.volume:
        name: my-volume
        location: fsn1
        size: 100
        state: present
      register: volume
```   

See the documentation and examples for more details.

##### ISSUE TYPE

- Feature Pull Request

docs/docsite/rst/authentification.rst

@@ -24,3 +24,25 @@ Alternatively, you may provide the API token directly as module argument:
         server_type: cx11
         image: debian-12
         state: present
+
+To reduce the duplication of the above solution, you may configure the
+``hetzner.hcloud.*`` modules using the ``hetzner.hcloud.all`` action group, for
+example if you want to store your API token in a vault:
+
+.. code-block:: yaml
+
+    - name: Demonstrate the usage of the 'hetzner.hcloud.all' module_defaults group
+      hosts: localhost
+      connection: local
+
+      module_defaults:
+        group/hetzner.hcloud.all:
+          api_token: "{{ _vault_hcloud_api_token }}"
+
+      tasks:
+        - name: Create server
+          hetzner.hcloud.server:
+            name: my-server
+            server_type: cx11
+            image: debian-12
+            state: present