Add link to the EUI docs to Kibana docs (#11835)

* Add 'Developing Plugins' section to the docs, with a link to the EUI Framework docs.

docs/development/plugin/development-plugin-resources.asciidoc

@@ -45,3 +45,10 @@ The Kibana directory must be named `kibana`, and your plugin directory must be l
 ==== References in the code
  - {repo}blob/{branch}/src/server/plugins/plugin.js[Plugin class]: What options does the `kibana.Plugin` class accept?
  - <<development-uiexports>>: What type of exports are available?
+
+[float]
+==== Elastic UI Framework
+If you're developing a plugin that has a user interface, take a look at our https://elastic.github.io/eui[Elastic UI Framework].
+It documents the CSS and React components we use to build Kibana's user interface.
+
+You're welcome to use these components, but be aware that they are rapidly evolving and we might introduce breaking changes that will disrupt your plugin's UI.

docs/gs-index.asciidoc

@@ -19,7 +19,6 @@ release-state can be: released | prerelease | unreleased
 :commit:         {repo}commit/
 :security:       https://www.elastic.co/community/security/
 
-
 include::introduction.asciidoc[]
 
 include::setup/install.asciidoc[]