feat: add page content type for site infrastructure

Distinct from articles/essays/notes — pages have no publish lifecycle,
no date, no TOC, no prev/next. <main> landmark, not <article>.
Groundwork: layouts/page/single.html + content/pages/about.md stub.

content/pages/about.md

@@ -0,0 +1,5 @@
+---
+title: "About"
+description: "What ASW is and who builds it."
+type: page
+---

layouts/page/single.html

@@ -0,0 +1,11 @@
+{{ define "header" }}{{ end }}
+
+{{ define "content" }}
+  <main>
+    <header>
+      <h1>{{ .Title }}</h1>
+      {{- with .Description }}<p>{{ . }}</p>{{ end }}
+    </header>
+    {{ .Content }}
+  </main>
+{{ end }}