docs: image rendering (#19183)

Adds a new "Image Rendering" page in Administration section.
Updates configuration page with rendering settings and also 
default.ini and sample.ini.
Updates and cleanup pages that referencing image rendering.

Ref #18914

docs/sources/administration/image_rendering.md

@@ -0,0 +1,51 @@
++++
+title = "Image Rendering"
+description = ""
+keywords = ["grafana", "image", "rendering", "phantomjs"]
+type = "docs"
+aliases = ["/installation/image-rendering"]
+[menu.docs]
+parent = "admin"
+weight = 8
++++
+
+# Image Rendering
+
+Grafana supports rendering of panels and dasnhboards as PNG-images.
+
+When an image is being rendered the PNG-image is temporary written to the filesystem, i.e. a sub-directory of Grafana's [data](/installation/configuration/#data) directory named `png`.
+
+A background job runs each 10 minutes and will remove temporary images. You can configure how long time an image should be stored before being removed by configuring the [temp-data-lifetime](/installation/configuration/#temp-data-lifetime) setting.
+
+## Rendering methods
+
+### PhantomJS
+
+> PhantomJS is deprecated since Grafana v6.4 and will be removed in a future release. Please migrate to Grafana image renderer plugin or remote rendering service.
+
+[PhantomJS](https://phantomjs.org/) have been the only supported and default image renderer since Grafana v2.x and is shipped with Grafana.
+
+Please note that for OSX and Windows, you will need to ensure that a phantomjs binary is available under tools/phantomjs/phantomjs. For Linux, a phantomjs binary is included - however, you should ensure that any required libraries, e.g. libfontconfig1, are available.
+
+### Grafana image renderer plugin
+
+The [Grafana image renderer plugin](https://grafana.com/grafana/plugins/grafana-image-renderer) is a plugin that runs on the backend and handles rendering panels and dashboards as PNG-images using headless chrome.
+
+You can install it using grafana-cli:
+
+```bash
+grafana-cli plugins install grafana-image-renderer
+```
+
+For further information and instructions refer to the [plugin details](https://grafana.com/grafana/plugins/grafana-image-renderer).
+
+### Remote rendering service
+
+The [Grafana image renderer plugin](https://grafana.com/grafana/plugins/grafana-image-renderer) can also be run as a remote HTTP rendering service. In this setup Grafana will render an image by making a HTTP request to the remote rendering service, which in turn render the image and returns it back in the HTTP response to Grafana.
+
+To configure Grafana to use a remote HTTP rendering service, please refer to [rendering](/installation/configuration/#rendering) configuration section.
+
+## Alerting and render limits
+
+Alert notifications can include images, but rendering many images at the same time can overload the server where the renderer is running. For instructions of how to configure this, see [concurrent_render_limit](/installation/configuration/#concurrent-render-limit).
+