[#676] Support Deeper Nesting in RTD Sidebar

This allows setting an arbitrary Sidebar Depth for both locally built
docs & docs on ReadTheDocs.org.

* Move the layout.html template to page.html since ReadTheDocs ignores
  the layout.html template.
* Reduce the Sidebar Depth to a level of 3 in order to reduce
  unnecessary entries in the Stencil Data Types & Behaviors Reference.

See: readthedocs/readthedocs.org#152

docs/source/_templates/layout.html → docs/source/_templates/page.html

@@ -1,7 +1,7 @@
-{% extends "!layout.html" %}
+{% extends "!page.html" %}
 
 {% block menu %}
-  {% set toctree = toctree(maxdepth=-1, collapse=True, includehidden=True) %}
+  {% set toctree = toctree(maxdepth=3, collapse=True, includehidden=True) %}
   {% if toctree %}
     {{ toctree }}
   {% else %}