Docs: Allow unlimited multi-level navigation (#1440)

* Remove `jekyll-default-layout` plugin

* Move docs/navigation-structure to docs/navigation

* Fix uses of line-nos in md files

* Update CHANGELOG.md

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

_config.yml

@@ -21,6 +21,14 @@ repository: just-the-docs/just-the-docs # for github-metadata
 
 permalink: pretty
 
+defaults:
+  -
+    scope:
+      path: "docs" # an empty string here means all files in the project
+      type: "pages"
+    values:
+      layout: "default"
+
 exclude:
  # from https://github.com/jekyll/jekyll/blob/master/lib/site_template/_config.yml:
    - .sass-cache/