added include template for images and refined documentation for alerts and callouts include templates

_includes/image.html

@@ -0,0 +1 @@
+<figure>{% if {{include.url}} %}<a class="no_icon" target="_blank" href="{{include.url}}">{% endif %}<img class="docimage" src="{{site.baseurl | prepend: site.url}}/images/{{include.file}}" alt="{{include.alt}}" {% if {{include.max-width}} %}style="max-width: {{include.max-width}}px"{% endif %} />{% if {{include.url}} %}</a>{% endif %}{% if {{include.caption}} %}<figcaption>{{include.caption}}</figcaption></figure>{% endif %}