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.

_includes/nav.html

 <nav role="navigation" aria-label="Main navigation">
   <ul class="navigation-list">
-    {%- assign pages_list = site.html_pages | sort:"nav_order" -%}
+    {%- assign ordered_pages_list = site.html_pages | where_exp:"item", "item.nav_order != nil" | sort:"nav_order" -%}
+    {%- assign unordered_pages_list = site.html_pages | where_exp:"item", "item.nav_order == nil" | sort:"title" -%}
+    {%- assign pages_list = ordered_pages_list | concat: unordered_pages_list -%}
     {%- for node in pages_list -%}
       {%- unless node.nav_exclude -%}
         {%- if node.parent == nil -%}
@@ -10,7 +12,7 @@
             {%- endif -%}
             <a href="{{ node.url | absolute_url }}" class="navigation-list-link{% if page.url == node.url %} active{% endif %}">{{ node.title }}</a>
             {%- if node.has_children -%}
-              {%- assign children_list = site.html_pages | where: "parent", node.title | sort:"nav_order" -%}
+              {%- assign children_list = pages_list | where: "parent", node.title -%}
               <ul class="navigation-list-child-list ">
                 {%- for child in children_list -%}
                   <li class="navigation-list-item {% if page.url == child.url or page.parent == child.title %} active{% endif %}">
@@ -19,7 +21,7 @@
                     {%- endif -%}
                     <a href="{{ child.url | absolute_url }}" class="navigation-list-link{% if page.url == child.url %} active{% endif %}">{{ child.title }}</a>
                     {%- if child.has_children -%}
-                        {%- assign grand_children_list = site.html_pages | where: "parent", child.title | sort:"nav_order" -%}
+                        {%- assign grand_children_list = pages_list | where: "parent", child.title -%}
                         <ul class="navigation-list-child-list">
                         {%- for grand_child in grand_children_list -%}
                           <li class="navigation-list-item {% if page.url == grand_child.url %} active{% endif %}">

_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 %}