Rename content slot to main, remove h1 from header for better

accessibility, and unify SVG fill color to #fff

Update BaseLayout to use a header <p> for the website title, rename the
main slot/id to
"main", and update all pages to match. Add CSS rule svg { fill:
currentColor }
and size header > p the same as h1. Fix a minor comment typo.

src/pages/index.astro

@@ -4,7 +4,7 @@ import Layout from "@layouts/BaseLayout.astro";
 
 <Layout>
   <title slot="head">Home</title>
-  <Fragment slot="content">
+  <Fragment slot="main">
     <article id="hero">
       <h2>Under Construction</h2>
       <p>Pardon the dust!</p>