Documentation: Add a tutorial on how to create a block-based theme

[carolinan]

It would be helpful to include a tutorial on how to create a block-based (FSE) theme
in the tutorial section at https://developer.wordpress.org/block-editor/tutorials/

Meeting notes related to updating the documentation for block-based themes:
https://make.wordpress.org/themes/2020/07/02/block-based-theme-meeting-notes-1-july-2020/