Fix liquid variable leakage in navigation components (#1243)

* Refactor nav, breadcrumbs, children_nav

Fix #1118

Improve the modularity of building the nav-panel, breadcrumbs, and children-nav
by making them independent. This also significantly simplifies the Liquid code.

* Fix order of breadcrumbs

* Update breadcrumbs.html

Revert inclusion of single breadcrumb for top-level pages.

* Update breadcrumbs.html

* Update children_nav.html

Revert to the previous layout in the HTML, to allow the use of `diff` to check the built site.

* Update minimal.html

Remove the previously required workaround involving `nav.html`.

* Add docs pages about layouts

The aim of the initial version of these docs pages is to illustrate the difference between the default and minimal layouts.

* Update CHANGELOG.md

_includes/sorted_pages.html

@@ -0,0 +1,95 @@
+{%- comment -%}
+  Include as: {%- include sorted_pages.html pages = pages -%}
+  Depends on: include.pages.
+  Assigns to: sorted_pages.
+  Overwrites: 
+    nav_order_pages, title_order_pages, 
+    nav_number_pages, nav_string_pages, nav_order_groups, group,
+    title_number_pages, title_string_pages, title_order_groups.
+{%- endcomment -%}
+
+{%- comment -%}
+  The `nav_order` values of pages affect the order in which they are shown in
+  the navigation panel and in the automatically generated tables of contents.
+  Sibling pages with the same `nav_order` value may be shown in any order.
+  Sibling pages with no `nav_order` value are shown after all pages that have
+  explicit `nav_order` values, ordered by their `title` values.
+  
+  The `nav_order` and `title` values can be numbers or strings. To avoid build
+  failures, we sort numbers and strings separately. We sort numbers by their
+  values, and strings lexicographically. The case-sensitivity of string sorting
+  is determined by the configuration setting of `nav_sort`. Pages with no `title`
+  value are excluded from the navigation.
+
+  Note: Numbers used as `title` or `nav_order` values should not be in quotes,
+  unless you intend them to be lexicographically ordered. Numbers are written
+  without spaces or thousands-separators. Negative numbers are preceded by `-`.
+  Floats are written with the integral and fractional parts separated by `.`.
+  (Bounds on the magnitude and precision are presumably the same as in Liquid.)
+{%- endcomment -%}
+
+{%- assign nav_order_pages = include.pages
+    | where_exp: "item", "item.nav_order != nil" -%}
+{%- assign title_order_pages = include.pages
+    | where_exp: "item", "item.nav_order == nil" -%}
+
+{%- comment -%}
+  Divide the arrays of `nav_order_pages` and `title_order_pages` according to
+  the type of value.
+  
+  The first character of the result of `jsonify` is `"` only for strings.
+  Grouping by a single character also ensures the number of groups is small.
+{%- endcomment -%}
+
+{%- assign nav_number_pages = "" | split: "" -%}
+{%- assign nav_string_pages = "" | split: "" -%}
+{%- assign nav_order_groups = nav_order_pages
+  | group_by_exp: "item", "item.nav_order | jsonify | slice: 0" -%}
+{%- for group in nav_order_groups -%}
+  {%- if group.name == '"' -%}
+    {%- assign nav_string_pages = group.items -%}
+  {%- else -%}
+    {%- assign nav_number_pages = nav_number_pages | concat: group.items -%}
+  {%- endif -%}
+{%- endfor -%}
+
+{%- unless nav_number_pages == empty -%}
+  {%- assign nav_number_pages = nav_number_pages | sort: "nav_order" -%}
+{%- endunless -%}
+
+{%- unless nav_string_pages == empty -%}
+  {%- if site.nav_sort == 'case_insensitive' -%}
+    {%- assign nav_string_pages = nav_string_pages | sort_natural: "nav_order" -%}
+  {%- else -%}
+    {%- assign nav_string_pages = nav_string_pages | sort: "nav_order" -%}
+  {%- endif -%}
+{%- endunless -%}
+
+{%- assign title_number_pages = "" | split: "" -%}
+{%- assign title_string_pages = "" | split: "" -%}
+{%- assign title_order_groups = title_order_pages
+    | group_by_exp: "item", "item.title | jsonify | slice: 0" -%}
+{%- for group in title_order_groups -%}
+  {%- if group.name == '"' -%}
+    {%- assign title_string_pages = group.items -%}
+  {%- else -%}
+    {%- assign title_number_pages = title_number_pages | concat: group.items -%}
+  {%- endif -%}
+{%- endfor -%}
+
+{%- unless title_number_pages == empty -%}
+  {%- assign title_number_pages = title_number_pages | sort: "title" -%}
+{%- endunless -%}
+
+{%- unless title_string_pages == empty -%}
+  {%- if site.nav_sort == 'case_insensitive' -%}
+    {%- assign title_string_pages = title_string_pages | sort_natural: "title" -%}
+  {%- else -%}
+    {%- assign title_string_pages = title_string_pages | sort: "title" -%}
+  {%- endif -%}
+{%- endunless -%}
+
+{%- assign sorted_pages = nav_number_pages
+    | concat: nav_string_pages
+    | concat: title_number_pages
+    | concat: title_string_pages -%}