Feature: Allow unlimited multi-level navigation (#1431)

* Allow unlimited multi-level navigation

This PR supersedes #462.

The only user-level difference from #462 is that disambiguation of parent pages has to use either `grand_parent` or `ancestor` titles: the somewhat unnatural `section_id` and `in_section` fields are not supported.

The implementation has been significantly simplified by the changes introduced in v0.7.0 of the theme.

* Detect cyclic parenthood

A page should not have a parent or ancestor with the same title. If it does, the location of the repeated link is marked by ∞, to facilitate debugging the navigation (and an unbounded loop leading to a build exception is avoided).

* Add nav_error_report warning in main navigation

When activated by `nav_error_report: true` in `_config.yml`, displays warnings about pages with the same title as their parent page or an ancestral page.

* Cache site-nav with links to all pages

The extra cached site-nav is used for determining breadcrumbs and children navigation, which may involve pages that are excluded from the main navigation.

* Replace code for determining children by inclusion of components/nav/children.html

* Update CHANGELOG.md

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Co-authored-by: Matt Wang <matt@matthewwang.me>

_includes/components/nav/pages.html

@@ -0,0 +1,23 @@
+{%- comment -%}
+  Include as: {%- include components/nav/pages.html pages=page_array all=bool -%}
+  Depends on: include.pages.
+  Results in: HTML for the main navigation when all is nil or false;
+    adds links to pages excluded from the main navigation when all is true.
+  Includes: components/nav/links.html
+  Assigns to:
+    nav_parenthood, nav_top_nodes, nav_top_node_titles, nav_ancestors.
+{%- endcomment -%}
+
+{%- assign nav_parenthood = include.pages
+      | where_exp: "item", "item.title != nil" | group_by: "parent" -%}
+
+{%- assign nav_top_nodes = nav_parenthood
+      | where_exp: "item", "item.name == ''" | map: "items" | first -%}
+
+{%- include components/nav/sorted.html pages=nav_top_nodes -%}
+
+{% assign nav_top_node_titles = nav_top_nodes | map: "title" -%}
+
+{%- assign nav_ancestors = "" | split: "" -%}
+
+{%- include components/nav/links.html pages=nav_sorted ancestors=nav_ancestors all=include.all -%}