feat: style homepage; add html head/body

I added some basic styles and spacing to the home page. The page size before this change is 5.85 kB The page size after this change is 6.63 kB The increase is pretty minimal but if you want to minimize the page size increase, you can include minified styles to bring the page size to 6.17 kB ```html <style>body{max-width:73%;margin:5% auto;font-family:sans-serif;color:#444;padding:0}h1,h2,h3{line-height:1.2}p{word-break:break-word}@media (prefers-color-scheme: dark){body{color:#c9d1d9;background:#0d1117}a:link{color:#58a6ff}a:visited{color:#8e96f0}}</style> ```