Add og meta tags and meta description

[adamziel]

Adds the following meta tags:

		<meta
			property="og:image"
			content="https://playground.wordpress.net/ogimage.png"
		/>
		<meta property="og:title" content="WordPress Playground" />
		<meta
			property="og:description"
			content="WordPress running in your browser for learning, testing, and developing with WordPress!"
		/>
		<meta property="og:url" content="https://playground.wordpress.net/" />
		<meta
			name="description"
			content="WordPress running in your browser for learning, testing, and developing with WordPress!"
		/>

I went with this image proposed by @jasmussen:

We can adjust it later on when the w.org redesign progresses.

Closes #960

CC @alexdeborba