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()
```

docs/docsite/rst/dev_guide/testing_validate-modules.rst

@@ -63,8 +63,8 @@ Errors
   **1xx**   **Locations**
   101       Interpreter line is not ``#!/usr/bin/python``
   102       Interpreter line is not ``#!powershell``
-  103       Did not find a call to ``main()``
-  104       Call to ``main()`` not the last line
+  103       Did not find a call to ``main()`` (or ``removed_module()`` in the case of deprecated & docs only modules)
+  104       Call to ``main()`` not the last line (or ``removed_module()`` in the case of deprecated & docs only modules)
   105       GPLv3 license header not found
   106       Import found before documentation variables. All imports must appear below
             ``DOCUMENTATION``/``EXAMPLES``/``RETURN``/``ANSIBLE_METADATA``