Code block feature guide should come with GIFs showcasing editing features

[oleq]

A follow-up of #5668.

GIF scenarios

Editing text around code blocks

Before

1. Use 1LOC code snippet.

   console.log( 'Hello World' );

2. Follow the scenario in the guide.

After

1. Use 1LOC code snippet.

   console.log( 'Hello World' );

2. Follow the scenario in the guide.

Changing line indentation

1. Use the following code snippet:

   	function log( message ) {
   	console.log( message );
   	}

2. Select the entire snippet.

3. Outdent.

4. Move the caret before the second line and indent it.

Preserving line indentation

1. Use the following code snippet:

   function log( message ) {
   	message = message.toLowerCase();
   }

2. Put the selection after message = message.toLowerCase();

3. Enter (new line created, preserved indent).

4. Type console.log( message ).