fix(layouts/index): Use <time> and <article>

layouts/index.html

 {{ define "main" -}}
 <div class="posts">
 {{ range .Data.Pages -}}
-<div class="post">
+<article class="post">
   <h1 class="post-title">
     <a href="{{ .Permalink }}">{{ .Title }}</a>
   </h1>
-  <span class="post-date">{{ .Date.Format "Mon, Jan 2, 2006" }}</span>
+  <time datetime="{{ .Date.Format "2006-01-02T15:04:05Z0700" }}" class="post-date">{{ .Date.Format "Mon, Jan 2, 2006" }}</time>
   {{ .Summary }}
   {{ if .Truncated }}
   <div class="read-more-link">
     <a href="{{ .RelPermalink }}">Read More…</a>
   </div>
   {{ end }}
-</div>
+</article>
 {{- end }}
 </div>
-{{- end }}
+{{- end }}