Lots of these were appearing:
```
*459 connect() failed (111: Connection refused) while connecting to upstream
```
This change resolves it, see https://stackoverflow.com/a/52550758
* [docs] Serve static assets with nginx
This explains how to use nginx to serve static assets and offload GTS
from that responsibility. It also shows how to have nginx add caching
headers to indicate to clients how long they may cache an asset.
* [docs] Move additional nginx config to advanced
This moves a bunch of additional nginx configuration into the Advanced
page instead. It declutters the nginx configuration page.
This explains how nginx can be used to cache webfinger responses and
potentially serve stale responses in case GTS is down. This can be
useful to do in order to ensure webfinger keeps working even if you're
doing some maintenance.
* 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
tobi