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/layout.md

@@ -0,0 +1,40 @@
+---
+title: Layout
+layout: default
+nav_order: 4.5
+has_children: true
+---
+
+# Layout
+
+You specify the layout for a page in its [front matter]. Just the Docs has a `default` layout with a sidebar, used for almost all pages in the theme docs, and a `minimal` layout that omits the sidebar.
+{: .fs-6 .fw-300 }
+
+## The layout concept
+
+See the [Jekyll docs page about layouts] for an explanation of the general idea of layouts and how to specify them.
+
+You can use [Jekyll's front matter defaults] to specify the same layout for many pages.
+
+## The `default` layout
+
+This page uses the default layout.
+
+It is a *responsive* layout: on medium and larger width displays, it displays a sidebar, including a navigation panel; on smaller width displays, the sidebar is automatically hidden under a button.
+
+Each child (and grandchild) page of a top-level page has so-called *breadcrumbs*: links to its parent (and grandparent) pages. It shows the breadcrumbs above the main content of the page.
+
+Each page that has child pages generally has a list of links to those pages (you can suppress it by `has_toc: false` in the front matter). It shows the list as a *table of contents* below the main content.
+
+## The `minimal` layout
+
+A child and grandchild page of this page use the minimal layout. This differs from the default layout by omitting the sidebar -- and thereby also the navigation panel. To navigate between pages with the minimal layout, you can use the breadcrumbs and the tables of contents.
+
+## Other layouts
+
+Just the Docs has further layouts: `about`, `home`, `page`, and `post`. Currently, they are all based on the `default` layout. See the [Jekyll docs about inheritance] for how to customize them.
+
+[front matter]: https://jekyllrb.com/docs/front-matter/ "Jekyll docs about front matter"
+[Jekyll docs page about layouts]: https://jekyllrb.com/docs/layouts/ "Jekyll docs about layouts"
+[Jekyll's front matter defaults]: https://jekyllrb.com/docs/configuration/front-matter-defaults/ "Jekyll docs about front matter defaults"
+[Jekyll docs about inheritance]: https://jekyllrb.com/docs/layouts/#inheritance "Jekyll docs about inheritance"

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.