Stabilise gradient theme API's

[jorgefilipecosta]

Description

This PR stabilizes the gradients theme API. And improves the documentation we have for these API's.

How has this been tested?

For each of the code blocks bellow I verified the result was the one described after each code block:

function test_code() {
	add_theme_support(
		'editor-gradient-presets',
		array(
			array(
				'name'     => __( 'Vivid cyan blue to vivid purple', 'themeLangDomain' ),
				'gradient' => 'linear-gradient(135deg,rgba(6,147,227,1) 0%,rgb(155,81,224) 100%)',
				'slug'     => 'vivid-cyan-blue-to-vivid-purple'
			),
			array(
				'name'     => __( 'Vivid green cyan to vivid cyan blue', 'themeLangDomain' ),
				'gradient' => 'linear-gradient(135deg,rgba(0,208,132,1) 0%,rgba(6,147,227,1) 100%)',
				'slug'     =>  'vivid-green-cyan-to-vivid-cyan-blue',
			),
			array(
				'name'     => __( 'Light green cyan to vivid green cyan', 'themeLangDomain' ),
				'gradient' => 'linear-gradient(135deg,rgb(122,220,180) 0%,rgb(0,208,130) 100%)',
				'slug'     => 'light-green-cyan-to-vivid-green-cyan',
			),
			array(
				'name'     => __( 'Luminous vivid amber to luminous vivid orange', 'themeLangDomain' ),
				'gradient' => 'linear-gradient(135deg,rgba(252,185,0,1) 0%,rgba(255,105,0,1) 100%)',
				'slug'     => 'luminous-vivid-amber-to-luminous-vivid-orange',
			),
			array(
				'name'     => __( 'Luminous vivid orange to vivid red', 'themeLangDomain' ),
				'gradient' => 'linear-gradient(135deg,rgba(255,105,0,1) 0%,rgb(207,46,46) 100%)',
				'slug'     => 'luminous-vivid-orange-to-vivid-red',
			),
		)
	);
	add_theme_support( 'disable-custom-gradients' );
}
add_action( 'after_setup_theme', 'test_code' );

The custom gradient picker is disabled. The gradient palette is changed to the one the theme is setting.

function test_code() {
	add_theme_support( 'editor-gradient-presets', array() );
	add_theme_support( 'disable-custom-gradients' );
}
add_action( 'after_setup_theme', 'test_code' );

The gradient functionality is tottaly disbaled.

function test_code() {
	add_theme_support( 'disable-custom-gradients' );
}
add_action( 'after_setup_theme', 'test_code' );

No default gradeints are provided the user can only use the custom gradient picker.