Explain opencloud specific requirements for a reverse proxy

[flimmy]

I repeatedly struggled to setup opencloud behind my own reverse proxies because I needed to search for the requirements or go by "trial and error".
Up until today I was unable to pin-point the exact issue that was holding me back, but talking with @micbar today and reviewing github-issues on the compose repo.

The Run behind ReverseProxy docs are more of a basic "nginx and letsencrypt"-HowTo, and not a documentation on how OpenCloud expects its reverse proxies to be configured.

I think this part of the docs is mainly used by advanced users or professional admins who do not want to use the compose-deployment we provide, because they already have a reverse proxy.
So they need information on how they need to configure their reverse proxy, and not a guide on how to set one up.

Sadly I don't have a "Patent-Rezept" on how to solve this yet :/

[flimmy]

Talking with @micbar again, I heard that the getting-started section should always contain a complete step-by-step guide.
Since this information doesn't fit into any of our other preexisting categories, I suppose we can add it to the guide on the bottom (adding a heading link on top of the page), or create a sub-page or sub-pages like:

• Behind External Proxy
  • Guide
  • DIY