Add DOI field button to bibliography (#2729)

_layouts/bib.liquid

@@ -192,6 +192,9 @@
       {% if entry.abstract %}
         <a class="abstract btn btn-sm z-depth-0" role="button">Abs</a>
       {% endif %}
+      {% if entry.doi %}
+        <a href="https://doi.org/{{ entry.doi }}" class="btn btn-sm z-depth-0" role="button">DOI</a>
+      {% endif %}
       {% if entry.arxiv %}
         <a href="http://arxiv.org/abs/{{ entry.arxiv }}" class="btn btn-sm z-depth-0" role="button">arXiv</a>
       {% endif %}