Alphabetic navigation order

When `nav_order` is omitted, the order of nodes at each menu level (and in the auto-generated TOC) is alphabetical by `title`, instead of random.
Any nodes with a specified `nav_order` precede all nodes at that level where it is omitted.
Note that `nav_order` fields must have a uniform site-ide type: integers and strings cannot be mixed, otherwise Jekyll reports errors.
The implementation filters the ordered and unordered pages from `site.html_pages`, sorts them separately, and concatenates the resulting arrays.

_layouts/default.html

@@ -74,9 +74,8 @@ layout: table_wrappers
           {% if page.has_children == true and page.has_toc != false %}
             <hr>
             <h2 class="text-delta">Table of contents</h2>
-            {% assign children_list = site.pages | sort:"nav_order" %}
             <ul>
-              {% for child in children_list %}
+              {% for child in pages_list %}
                 {% if child.parent == page.title and child.title != page.title %}
                 <li>
                   <a href="{{ child.url | absolute_url }}">{{ child.title }}</a>{% if child.summary %} - {{ child.summary }}{% endif %}