Module deprecation: docs, scheme and tests (#34100)

Enforce module deprecation.
After module has reached the end of it's deprecation cycle we will replace it with a docs stub.

* Replace deprecated modules with docs-only sub
* Use of deprecated past deprecation cycle gives meaningful message (see examples below)
* Enforce documentation.deprecation dict via `schema.py`
* Update `ansible-doc` and web docs to display documentation.deprecation
* Document that structure in `dev_guide`
* Ensure that all modules starting with `_` have a `deprecation:` block
* Ensure `deprecation:` block is only used on modules that start with `_`
* `removed_in` A string which represents when this module needs **deleting**
* CHANGELOG.md and porting_guide_2.5.rst list removed modules as well as alternatives
* CHANGELOG.md links to porting guide index

To ensure that meaningful messages are given to the user if they try to use a module at the end of it's deprecation cycle we enforce the module to contain:
```python
if __name__ == '__main__':
    removed_module()
```

lib/ansible/modules/clustering/k8s/_kubernetes.py

@@ -14,7 +14,10 @@ DOCUMENTATION = '''
 ---
 module: kubernetes
 version_added: "2.1"
-deprecated: In 2.5 use M(k8s_raw) instead.
+deprecated:
+  removed_in: "2.9"
+  why: This module used the oc command line tool, where as M(k8s_raw) goes over the REST API.
+  alternative: Use M(k8s_raw) instead.
 short_description: Manage Kubernetes resources
 description:
     - This module can manage Kubernetes resources on an existing cluster using