docs: Update proxy docs.
docs: Update proxy docs.

Notable changes:
 - Describe `X-Forwarded-For` by name.
 - Switch each specific proxy to numbered steps.
 - Link back to the `X-Forwarded-For` section in each proxy
 - Default to using HTTPS, not HTTP, for the backend.
 - Include the HTTP-to-HTTPS redirect code for all proxies; it is
   important that it happen at the proxy, as the backend is unaware of
   it.
 - Call out Apache2 modules which are necessary.
 - Specify where the dhparam.pem file can be found.
 - Call out the `Host:` header forwarding necessary, and document
   `USE_X_FORWARDED_HOST` if that is not possible.
 - Standardize on 20 minutes of connection timeout.

zulip/zulip,zulip/zulip,rht/zulip,zulip/zulip,andersk/zulip,rht/zulip,rht/zulip,zulip/zulip,rht/zulip,andersk/zulip,rht/zulip,andersk/zulip,andersk/zulip,andersk/zulip,rht/zulip,zulip/zulip,andersk/zulip,andersk/zulip,zulip/zulip,rht/zulip,zulip/zulip