TINY-590: move to advanced topics and rename

_data/nav.yml

@@ -398,6 +398,7 @@
   - url: "accessibility"
   - url: "security"
   - url: "boilerplate-content-css"
+  - url: "creating-a-custom-button"
   - url: "creating-a-skin"
   - url: "creating-a-plugin"
   - url: "creating-custom-dialogs"

advanced/creating-a-custom-button.md

@@ -1,6 +1,6 @@
 ---
 layout: default
-title: Howto create custom toolbar button
+title: Create a Custom Toolbar Button
 title_nav: Custom Toolbar Button
 description_short: Add a custom button to the toolbar.
 description: This example shows you how to add a custom button to the toolbar.