* Add instructions for working with Caddy 2
Add instructions for working with Caddy 2. Some texts are duplicated from the NGINX part (mainly the configuration part, about systemctl).
* Add new Caddy docs to mkdocs.yml
Adds the new Caddy document to the document tree
* Remove up and downstream headers from configuration
Removed the header statements for the proxy, as proxying those are transparant. Kept the flush_interval directive, disabling the response buffer completely so we can write without delays.
* Update caddy.md
Corrects the opening link
* Apply comments mentioned in PR
@igalic mentioned a few comments to improve these docs. I've processed those in this PR.
