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()
```

test/units/modules/cloud/docker/test_docker.py

@@ -1,20 +0,0 @@
-import collections
-import os
-import unittest
-
-from ansible.modules.cloud.docker._docker import get_split_image_tag
-
-
-class DockerSplitImageTagTestCase(unittest.TestCase):
-
-    def test_trivial(self):
-        self.assertEqual(get_split_image_tag('test'), ('test', 'latest'))
-
-    def test_with_org_name(self):
-        self.assertEqual(get_split_image_tag('ansible/centos7-ansible'), ('ansible/centos7-ansible', 'latest'))
-
-    def test_with_tag(self):
-        self.assertEqual(get_split_image_tag('test:devel'), ('test', 'devel'))
-
-    def test_with_tag_and_org_name(self):
-        self.assertEqual(get_split_image_tag('ansible/centos7-ansible:devel'), ('ansible/centos7-ansible', 'devel'))