Docs: Add guidelines for running grafana behind proxy on RPM-based Linux (#41710)

* Docs: Add Centos guidelines for running grafana behind proxy

* Apply suggestions from code review

Co-authored-by: achatterjee-grafana <70489351+achatterjee-grafana@users.noreply.github.com>

docs/sources/installation/rpm.md

@@ -172,6 +172,30 @@ sudo systemctl enable grafana-server
 
 {{< docs/shared "systemd/bind-net-capabilities.md" >}}
 
+#### Serving Grafana behind a proxy
+
+When serving Grafana behind a proxy, you need to configure the `http_proxy` and `https_proxy` environment variables.
+
+##### Centos 6
+
+If you are on Centos 6, add the following lines to the `/etc/sysconfig/grafana-server` file.
+
+```
+export no_proxy=internal.domain,127.0.0.1
+export http_proxy=http://proxy.domain:3128/
+export https_proxy=http://proxy.domain:3128/
+```
+
+##### Centos 7
+
+If you are on Centos 7, add the following lines to the `/etc/sysconfig/grafana-server` file.
+
+```
+http_proxy=http://proxy.domain:3128/
+https_proxy=http://proxy.domain:3128/
+no_proxy=internal.domain,127.0.0.1
+```
+
 ### Start the server with init.d
 
 To start the service and verify that the service has started: