cv: Add overview nav link

This will provide a way for users to return to the "index" page of the
CV from one of the career pages.

templates/cv-nav.html

@@ -1,5 +1,6 @@
 <nav class="cv-nav">
 <ul class="cv-nav">
+  <li {% if page is not defined %}class="active"{% endif %}><a href="{{ section.permalink | safe }}">Overview</a></li>
 {% for p in section.pages %}
   <li {% if page is defined and p.slug == page.slug %}class="active"{% endif %}><a href="{{ p.permalink | safe }}">{{ p.title }}</a></li>
 {% endfor %}