Add header block for Google's 'Rich Snippets' nonsense

Not content with or [News Industry Text Format](https://www.iptc.org/standards/nitf/), or Facebook/Meta's ['Open Graph'](https://ogp.me/) syntax to generate pretty banners for URLs, or Twitter/X's similar-but-different mechanism, or any other existing standard, Google have had to reinvent the wheel in a [completely different way](https://developers.google.com/search/docs/appearance/structured-data/article#json-ld). Because of course they do.

And while I hate this perverse 'not invented here' proliferation of competing standards all doing more or less the same thing and all adding bloat to web pages, given that Cryogen already does support both the Facebook and Twitter standards (and I'm largely responsible for this), it seems foolish not to add the Google version as well.

Therefore I propose to do so unless other people object; expect a pull request shortly.