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

docs/layout/minimal/default-child.md

@@ -0,0 +1,8 @@
+---
+title: Default layout child page
+layout: default
+parent: A minimal layout page 
+grand_parent: Layout
+---
+
+This is a child page that uses the same minimal layout as its parent page.

docs/layout/minimal/minimal-child.md

@@ -0,0 +1,8 @@
+---
+title: Minimal layout child page
+layout: minimal
+parent: A minimal layout page
+grand_parent: Layout
+---
+
+This is a child page that uses the same minimal layout as its parent page.

docs/layout/minimal/minimal.md

@@ -0,0 +1,12 @@
+---
+title: A minimal layout page
+layout: minimal
+parent: Layout
+has_children: true
+---
+
+# A minimal layout page
+
+This page illustrates the built-in layout `minimal`.
+
+One of its child pages also uses the minimal layout; the other child pages uses the default layout.