Simplify assigning classname to docs' aside-links (#7609)

Merge pull request 7609

docs/_includes/docs_contents.html

@@ -5,11 +5,7 @@
       <ul>
         {% for item in section.docs %}
           {% assign p = site.documents | where: "url", item.link | first %}
-          <li class="{% if p.url == '/docs/' %}
-            {% if page.url == '/docs/' %}current{% endif %}
-          {% else %}
-            {% if page.url contains p.url %}current{% endif %}
-          {% endif %}">
+          <li {%- if page.url == p.url %} class="current" {%- endif %}>
             <a href="{{ p.url }}">
               {{ p.menu_name | default: p.title }}
             </a>