No mention of core version of theme.json Schema in living reference

[DaisyOlsen]

What problem does this address?

There is no reference to the core version specific schema URLs in the theme.json version 2 iving reference.

What is your proposed solution?

Expand on the schema section of this page to indicate these versions for use when core WordPress is being used without the Gutenberg plugin.

[mburridge]

Add schema and version info to page:

{
	"$schema": "https://schemas.wp.org/trunk/theme.json",
    "version": 2

Also, clean up text at top of page.

[justintadlock]

Current Theme Handbook reference for this: https://developer.wordpress.org/themes/advanced-topics/theme-json/#how-to-create-a-theme-json-file