since 2.4.47 (released April 22nd 2021), Apache httpd can ProxyPass to
websockets on the same URL, without mod_rewrite (and, without
mod_proxy_wstunnel).
* 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
