Draft: Better home page

There's a few changes that have been added in this MR, mostly dealing with the home page:

• Rather than a second call-to-action to "get started", I referenced the basic concepts page with "Learn Shpadoinkle", with ("or, get started") underneath it
• The feature cards have better prose, emphasis, and boldness where applicable
• The feature cards have links to "learn more", referencing appropriate articles
• (TODO) Reliability and Type Safety features have their own articles
• Fast references the Backends page
• Declarative references Basic Concept
• Composable references both Lenses and the Calculator tutorial
• Reliable references (TODO)
• Type Safe references Router and (TODO)
• Small prose fixes for the components

This is in reference to issue #50