guides: hide guides with skip_listing: true

Adds a frontmatter attribute that hides an entry from being listed
in the main guides page.

src/_components/guides/category/section.rb

@@ -7,9 +7,9 @@ def initialize(category:, title: nil, css_classes: "")
     @category_name = category.name
     @description = category.description
     @resources = @site.collections.guides.resources.select do |guide|
-      guide.data.categories.find do |category|
+      !guide.data.skip_listing && guide.data.categories.find { |category|
         Bridgetown::Utils.slugify(category) == Bridgetown::Utils.slugify(@category_name)
-      end
+      }
     end
     @remaining_count = @resources.count - 3
   end

src/_guides/openapi/specification/_defaults.yml

@@ -1,3 +1,4 @@
 layout: indepth_guide
 page_class: guide indepth-guide with-sidebar
 sidebar_title: OpenAPI Specification
+skip_listing: true