Reorganize Landing pages for Shipwright website

[adambkaplan]

Thanks for the ping! Perhaps unsurprisingly, I do indeed have thoughts about this 😄

I think a good README or landing page follows roughly the form:

1. Here's what it is, in a sentence or two ("Framework for building container images on Kubernetes" is great 👍)
2. Here's what it can do for you -- here's how easy your life will be if you try it
3. Here's how to start using it -- a simple mini-tutorial, with links to deeper tutorials
4. Here's how it works, if you're interested -- maybe just a link to architecture.md
5. Here's how to get involved, if you'd like to contribute

Each step draws the reader in a little deeper, and the user can stop at whatever level they want if they don't care about the next level. (FWIW, Tekton's docs also aren't perfect here, but we try -- Shipwright's API is a lot simpler than Tekton's, so I think we can make the docs really smooth)

"How it works" is purposefully low on that list. Users don't care and shouldn't care how it works (until it doesn't 😅 ), but a lot of docs written by engineers want to jump straight there.

I might send some docs PRs to try to achieve this, if this outline sounds good.

Originally posted by @imjasonh in shipwright-io/build#441 (comment)