Image rendering instructions for PMM

The problem: 

We can't include Image rendering in PMM server as for now by default. 

But users need to be provided with clear steps and instructions on how to make image rendering working.

More details: https://jira.percona.com/browse/PMM-5735?focusedCommentId=266984&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-266984 

Proposed solution:

• Create a specific page in the PMM Documentation about "how to make image rendering  working on PMM" 
  • mention technical steps to install all needed packages
  • the command to install the plugin. 
  • with no changes in documentation? ( vadim.yalovets  pls confirm)
  • Warn user about Increased disc size (from our investigation, see related issue ) and CPU recommendation (from grafana doc ) 

Additional links:

• https://grafana.com/docs/grafana/latest/administration/image_rendering/#image-rendering