Made case-insenstive sorting the default

Added a configuration option to determine whether the sort order is case-sensitive.
The default is case-insensitive.

To test:
- open `/just-the-docs/docs/utilities/` in the browser,
  and check that the navigation links in `Utilities` are sorted alphabetically;
- in `docs/utilities/layout.md', change the preamble to `title: layout`,
  and check that the  links in `Utilities` are still sorted alphabetically;
- add `nav_sort: case_sensitive` in the configuration file,
  and check that the link to `layout` is now listed last under `Utilities`.

_includes/nav.html

@@ -1,8 +1,15 @@
 <nav role="navigation" aria-label="Main navigation">
   <ul class="navigation-list">
-    {%- 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 -%}
+    {%- assign ordered_pages_list = site.html_pages | where_exp:"item", "item.nav_order != nil" -%}
+    {%- assign unordered_pages_list = site.html_pages | where_exp:"item", "item.nav_order == nil" -%}
+    {%- if site.nav_sort == 'case_sensitive' -%}
+      {%- assign sorted_ordered_pages_list = ordered_pages_list | sort:"nav_order" -%}
+      {%- assign sorted_unordered_pages_list = unordered_pages_list | sort:"title" -%}
+    {%- else -%}
+      {%- assign sorted_ordered_pages_list = ordered_pages_list | sort_natural:"nav_order" -%}
+      {%- assign sorted_unordered_pages_list = unordered_pages_list | sort_natural:"title" -%}
+    {%- endif -%}
+    {%- assign pages_list = sorted_ordered_pages_list | concat: sorted_unordered_pages_list -%}
     {%- for node in pages_list -%}
       {%- unless node.nav_exclude -%}
         {%- if node.parent == nil -%}