UI improvements

[freeman-lab]

We now have a very basic UI for the landing page. While it's functional, it revealed a couple improvements we likely want to make.

• Decide what metadata we want to show on each card (Presumably more than what I have here, which is just the name, a brief description, and the primary coding language) And ideally we often (or always) source this metadata from GitHub rather than hard code it? That would make sense if, almost always, each card refers to a GitHub repo.
• Make sure subpages have some kind of "Back to Docs" button. As it is, you can easily navigate to a Project subpage and then find it hard to get back to the Docs landing page (it's easy to get back to the CarbonPlan homepage or other primary CarbonPlan pages, but, that might not be what someone wants). Any fix there will have to happen on each subpage, rather than here in the landing site, but I'm raising here as it's a UI quirk that affects the full experience.
• We're not using social cards at all here. We should design one for the main Docs site, and then, ideally, have automated cards generated for the individual sites, a la GitHub.

Those feel top priority to me, but feel free to add others!