landmark structure: header/main/footer as direct children of body

Per Pico CSS landmark pattern and HTML5 spec:
- Site <header> with <nav> moved to baseof.html
- Site <footer> moved to baseof.html
- All section layouts (single, list, docs, paper, vault, index)
  now only define <main> content — no duplicate nav/footer
- Article <header> stays inside <article> (different semantic role)

layouts/paper/single.html

@@ -1,6 +1,4 @@
 {{ define "main" }}
-  {{ partial "nav.html" . }}
-
   <main>
     <article data-paper>
 
@@ -58,8 +56,4 @@
 
     </article>
   </main>
-
-  <footer>
-    <small><a href="/">{{ .Site.Title }}</a> · {{ .Date.Format "2006" }}</small>
-  </footer>
 {{ end }}