docs: UX improvements

[metonym]

• Add favicon
• Make footer sticky
• Serialize search query to URL params, making searches share-able
• Make LineNumbers example look better
• Switch to ir-black as the default theme (it's higher contrast, and thus looks a bit better on a dark UI)

[render[bot]]

Your Render PR Server URL is https://svelte-highlight-pr-306.onrender.com.

Follow its progress at https://dashboard.render.com/static/srv-clr5fh41jt2c73enlrfg.