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/cloud/azure/_azure.py

@@ -19,7 +19,10 @@ short_description: create or terminate a virtual machine in azure
 description:
      - Creates or terminates azure instances. When created optionally waits for it to be 'running'.
 version_added: "1.7"
-deprecated: "Use M(azure_rm_virtualmachine) instead."
+deprecated:
+  removed_in: "2.8"
+  why: Replaced with various dedicated Azure modules.
+  alternative: M(azure_rm_virtualmachine)
 options:
   name:
     description: