add Figma embed block reference

[jeffpaul]

What does the code do?

This PR adds an example in the Figma-to-blocks section of the Figma embed block plugin.

Code Checklist

• It contains a readme.md in the root directory.
  • The readme explains how to use the tool.
  • The readme explains how to develop the tool.
• This code has a GPL-compatible license.
• This code follows [plugin guidelines](https://developer.wordpress.org/plugins/wordpress-org/detailed-plugin-guidelines/
  #the-guidelines).
• I am the original author of this code.

[jeffpaul]

@jordesign checking to see if there's anything additional needed on this PR?