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/cli/doc.py

@@ -468,7 +468,7 @@ class DocCLI(CLI):
         if 'deprecated' in doc and doc['deprecated'] is not None and len(doc['deprecated']) > 0:
             text.append("DEPRECATED: \n")
             if isinstance(doc['deprecated'], dict):
-                text.append("\tReason: %(why)s\n\tScheduled removal: Ansible %(version)s\n\tAlternatives: %(alternative)s" % doc.pop('deprecated'))
+                text.append("\tReason: %(why)s\n\tWill be removed in: Ansible %(removed_in)s\n\tAlternatives: %(alternative)s" % doc.pop('deprecated'))
             else:
                 text.append("%s" % doc.pop('deprecated'))
             text.append("\n")