Documentation: Document all available script handles

[aduth]

Discussed today in the Slack #core-js Weekly Chat (link requires registration)

Gutenberg registers a number of scripts:

• Each package
• A number of "vendor" dependencies

Use of these scripts is not well documented.

Task: Create new documentation under docs/, to be included in the "Reference" section of the Handbook, detailing which script handles are available for plugin authors to reference.