Show Infinite Depth in Sidebar of Docs

Create a Sphinx template fragment to remove the maximum depth of
sections shown & to collapse children of sibling sections.

Refs #676

docs/source/_templates/layout.html

@@ -0,0 +1,11 @@
+{% extends "!layout.html" %}
+
+{% block menu %}
+  {% set toctree = toctree(maxdepth=-1, collapse=True, includehidden=True) %}
+  {% if toctree %}
+    {{ toctree }}
+  {% else %}
+    <!-- Local TOC -->
+    <div class="local-toc">{{ toc }}</div>
+  {% endif %}
+{% endblock %}