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/setup.py

@@ -18,11 +18,6 @@ install_requires = [
     'zope.interface',
 ]
 
-docs_extras = [
-    'Sphinx>=1.0',  # autodoc_member_order = 'bysource', autodoc_default_flags
-    'sphinx_rtd_theme',
-]
-
 
 class PyTest(TestCommand):
     user_options = []
@@ -74,9 +69,6 @@ setup(
     packages=find_packages(),
     include_package_data=True,
     install_requires=install_requires,
-    extras_require={
-        'docs': docs_extras,
-    },
     entry_points={
         'certbot.plugins': [
             'nginx = certbot_nginx.configurator:NginxConfigurator',