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/tools.astro

@@ -13,7 +13,7 @@ import Layout from "@layouts/BaseLayout.astro";
       margin: 2rem auto 0;
     }
   </style>
-  <Fragment slot="content">
+  <Fragment slot="main">
     <section class="tools" id="tools">
       <h2>Tools</h2>
       <div class="tools-grid">