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

CHANGELOG.md

@@ -8,6 +8,11 @@ Certbot adheres to [Semantic Versioning](https://semver.org/).
 
 *
 
+### Removed
+
+* The `docs` extras for the `certbot-apache` and `certbot-nginx` packages
+  have been removed.
+
 ### Changed
 
 * certbot-auto has deprecated support for systems using OpenSSL 1.0.1 that are
@@ -87,7 +92,7 @@ More details about these changes can be found on our GitHub repo.
 ### Changed
 
 * Don't send OCSP requests for expired certificates
-* Return to using platform.linux_distribution instead of distro.linux_distribution in OS fingerprinting for Python < 3.8 
+* Return to using platform.linux_distribution instead of distro.linux_distribution in OS fingerprinting for Python < 3.8
 * Updated the Nginx plugin's TLS configuration to keep support for some versions of IE11.
 
 ### Fixed