Add metadata block

[AshGDS]

⚠️ This repo is Continuously Deployed: make sure you follow the guidance ⚠️

What / Why

• Adds a metadata block for the landing page
• These meta tags make social share previews look better
• It grabs the title and description from the content item by default, but these can be overridden in the yml body.
• You can use the Social share preview plugin to see how it will look to users
• The Machine readable data component crashes when I import it into PfG, and doesn't allow featured image customisation, so I've just added the metatags using a block. We've done similar for the GOVUK Roadmap
• Trello card

Screenshots

[andysellick]

Looks fine to me, my only question is should this be a block that we need to remember to add, or should it be built into the page template somehow?

[AshGDS]

Thanks @andysellick Probably, but I think that would require some backend work - should I get one of the backend devs to help with this card?

[andysellick]

@AshGDS I'm prepared to be wrong on this one, so maybe just start with a chat with a BE dev and see what they think.

[andysellick]

Approving for now but may find a different solution later.