Remove unused nginx docs (#7576)

Part of #5775. We don't use these docs anywhere, so delete them.

Removes:
- `certbot-nginx/readthedocs.org.requirements.txt`
- `certbot-nginx/docs/` folder
- docs include in `MANIFEST.in`
- docs dependencies in `setup.py`

* Remove unused nginx docs

* Add changelog entry about the removal

certbot-nginx/MANIFEST.in

@@ -1,5 +1,4 @@
 include LICENSE.txt
 include README.rst
-recursive-include docs *
 recursive-include certbot_nginx/tests/testdata *
 recursive-include certbot_nginx/tls_configs *.conf