Change some headers back to Lutschine

src/components/MDX/H1.astro

@@ -1 +1 @@
-<h1 class="text-3xl font-extrabold"><slot /></h1>
+<h1 class="font-header text-3xl"><slot /></h1>

src/components/MDX/H2.astro

@@ -1 +1 @@
-<h2 class="text-2xl font-extrabold"><slot /></h2>
+<h2 class="font-header text-2xl"><slot /></h2>

src/components/SectionTitle.astro

@@ -15,6 +15,6 @@ const {
 ---
 
 <div class={className}>
-  <Tag class="m-0 text-center text-3xl font-extrabold"><slot /></Tag>
+  <Tag class="font-header m-0 text-center text-3xl"><slot /></Tag>
   <hr class={`mx-auto my-4 w-16 border-2 ${lineColour} ${lineColourDark}`} />
 </div>