[aicayzer]

This pull request polishes the site, mostly for phones.

• Phones: smaller text, tighter spacing and a narrower gutter below 560px, so the home page, its questions included, fits on one screen of a typical iPhone. The What's next periods stay beside their steps.
• Header: a translucent Docs pill with an arrow, 26px tall with 14px corners, at the top right of every page. Pages other than home show "Marfa" at the top left as the way back.
• Footer: About, Contact and GitHub. Contact opens an email to hello@withmarfa.com.
• About: a short "Get in touch" section with the same address.
• Polish: text settles in on load (off with reduced motion), questions open and close smoothly where the browser supports it, a drawn plus and minus, balanced headings, and safe-area padding at the bottom.