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/network/citrix/_netscaler.py

@@ -18,7 +18,10 @@ version_added: "1.1"
 short_description: Manages Citrix NetScaler entities
 description:
      - Manages Citrix NetScaler server and service entities.
-deprecated: In 2.4 use M(netscaler_service) and M(netscaler_server) instead.
+deprecated:
+  removed_in: "2.8"
+  why: Replaced with Citrix maintained version.
+  alternative: Use M(netscaler_service) and M(netscaler_server) instead.
 options:
   nsc_host:
     description: