a11y: add skipnav link

layouts/_default/baseof.html

@@ -36,7 +36,7 @@
   <header>
     {{- partial "header.html" . -}}
   </header>
-  <main>
+  <main id="maincontent">
     {{- block "main" . }}{{- end }}
   </main>
   <footer>

layouts/partials/header.html

@@ -1,2 +1,4 @@
+<a class="skiplink" href="#maincontent">Skip to Main Content</a>
+
 <a href="{{ relURL .Site.Home.Permalink }}" class="title"><h1>{{ .Site.Title }}</h1></a>
 <nav>{{- partial "nav.html" . -}}</nav>