Add a 'Compatibility' or 'What's Changed' page to the handbook

[noisysocks]

It would be nice to have a dedicated page or section in the handbook that documents all of the potentially breaking changes between the Classic Editor and Gutenberg.

Some things that this page might cover:

• Changes to the editor UI that are intentional – see Document known changes in editor behavior #4186
• Gutenberg equivalents for each of the Classic Editor extensibility endpoints – see Document WordPress classic editor integration points and Gutenberg equivalents #4151
• Introduction of the wp_blocks Custom Post Type and /wp/v1/blocks endpoint – see Add ability to delete a Reusable Block to REST API #4137 (review)

[karmatosed]

http://wordpress.org/gutenberg is a good start with the section on compatibility. That's not saying we shouldn't have a handbook section, but we can begin there.

[mtias]

Let's fold this into #4186.