Add a way to add OpenGraph tags

https://ogp.me/

OpenGraph allows us to specify preview images and other stuff that makes social media links to our pages better. Here's how we've used them on the standalone Connect site (which I'm working on just rolling into our Jekyll site):

https://gitlab.com/CentOS/promo/connect/-/blob/master/index.html?ref_type=heads

I'd like to have tags like

<meta property="og:image:url" content="path://to/image.png"></meta>

And maybe we just get that from something in the YAML frontmatter

og_image_url: "path://to/image.png"