docs: how to add "Copy" button to code blocks (#1047)

* Update guides-custom-pages.md

Guide on how to add "Copy" to clipboard buttons on code blocks in Docusaurus.

* Update guides-custom-pages.md

Changed wall of text to be a link to a gist describing the code you need to add to add copy clipboard

* Add to markdown instead

* Update guides-custom-pages.md

docs/api-doc-markdown.md

@@ -206,3 +206,7 @@ class Example extends React.Component {
   }
 }
 ```
+
+### Adding Copy Code Buttons
+
+Docusaurus allows for adding buttons to copy code within fenced code blocks. Please follow the instructions [here](https://gist.github.com/yangshun/55db997ed0f8f4e6527571fc3bee4675) to add "Copy" buttons to your code blocks.