[docs] unify nginx explainers and add apache httpd (#455)

* docs: unify nginx explainers and add apache httpd

there are two places where nginx + certbot is explained, unify that into
one place.
Add apache httpd, following the same steps, but using mod_md for
LetsEncrypt
add a note about #453 in both guides.
Link to both, and call the section reverse proxy, instead of NGINX

* restore full nginx.conf from docker.md

* add installation_guide/apache-httpd.md to mkdocs

mkdocs.yml

@@ -18,6 +18,7 @@ nav:
     - "installation_guide/binary.md"
     - "installation_guide/docker.md"
     - "installation_guide/nginx.md"
+    - "installation_guide/apache-httpd.md"
     - "installation_guide/third_party.md"
   - "Configuration":
     - "configuration/index.md"