Dashboard image sharing (Share Panel): Improved wording with link to configuration instructions

The problem:  Current instructions for how to enable image rendering are for Grafana but not for PMM

Proposed solution: 

When  PMM-6597 implemented we should change the text

"To render a panel image, you must install the Grafana Image Renderer plugin. Please contact your Grafana administrator to install the plugin."

Proposed solution: replace the text with:  

To render a panel image, you must install the Image Renderer plugin. Please contact your PMM administrator to install the plugin.

Link: 

https://www.percona.com/doc/percona-monitoring-and-management/2.x/metrics-monitor-dashboards.html#rendering-dashboard-images