Improve docs about X-Forwarded-* headers and move it to nginx.md

[aradalvand]

The documentation about customizing the X-Forwarded-* headers that Nginx sends to the app (e.g. dokku nginx:set app x-forwarded-for-value '$http_x_forwarded_for') was previously located in the documentation page for SSL, but it really has nothing to do with SSL. I think this must've been a mistake, it actually belongs to the Nginx documentation, the command is literally dokku nginx, and these headers aren't specifically related to SSL.

I also made some improvements to this part of the documentation. For example, previously the docs here said that you're supposed to customize these headers if your server is running behind a load balancer, but this applies more generically, to any type of HTTP proxy that might be present in front of your Dokku server, a load balancer is just one type of proxy, this is not limited to load balancers, it also applies to CDN edge servers, for instance, and so on.

Another minor thing is that previously the command mentioned in the docs looked like this:

dokku nginx:set node-js-app x-forwarded-for-value "\$http_x_forwarded_for"

While it could just be:

dokku nginx:set node-js-app x-forwarded-for-value '$http_x_forwarded_for'

There's no need to use double quotes over single quotes just to then escape the $. You should just use single quotes when you don't want string interpolation.

I also reworded some parts of the explanation to make it more comprehensive and easily understandable.

[josegonzalez]

Mind adding a link to these docs at the bottom of the page as we do here in the nginx docs?

[aradalvand]

@josegonzalez Sure! Done.