Improve dom_id uniqueness in guides

All headers in a guide get a unique `dom_id` to make anchor links work.
If a header is already present we would prefix it with the `dom_id` of
the parent node. This would not work for headers without parent nodes.

This commit simplifies the `dom_id` uniqueness by only prepending the
parent node if it exists. This can still result in duplicates at the
same level, but for these we already show a warning:

    *** DUPLICATE ID: 'some_id', please make sure that there are no
    headings with the same name at the same level.

Co-authored-by: Carlos Antonio da Silva <carlosantoniodasilva@gmail.com>

guides/rails_guides/generator.rb

@@ -177,7 +177,7 @@ def extract_anchors(html)
         anchors = Set.new
         html.scan(/<h\d\s+id="([^"]+)/).flatten.each do |anchor|
           if anchors.member?(anchor)
-            puts "*** DUPLICATE ID: #{anchor}, please make sure that there are no headings with the same name at the same level."
+            puts "*** DUPLICATE ID: '#{anchor}', please make sure that there are no headings with the same name at the same level."
           else
             anchors << anchor
           end

guides/rails_guides/markdown.rb

@@ -35,16 +35,15 @@ def render(body)
       def dom_id(nodes)
         dom_id = dom_id_text(nodes.last.text)
 
-        # Fix duplicate node by prefix with its parent node
+        # Fix duplicate dom_ids by prefixing the parent node dom_id
         if @node_ids[dom_id]
           if @node_ids[dom_id].size > 1
             duplicate_nodes = @node_ids.delete(dom_id)
-            new_node_id = "#{duplicate_nodes[-2][:id]}-#{duplicate_nodes.last[:id]}"
+            new_node_id = dom_id_with_parent_node(dom_id, duplicate_nodes[-2])
             duplicate_nodes.last[:id] = new_node_id
             @node_ids[new_node_id] = duplicate_nodes
           end
-
-          dom_id = "#{nodes[-2][:id]}-#{dom_id}"
+          dom_id = dom_id_with_parent_node(dom_id, nodes[-2])
         end
 
         @node_ids[dom_id] = nodes
@@ -60,6 +59,14 @@ def dom_id_text(text)
                      .gsub(/\s+/, "-")
       end
 
+      def dom_id_with_parent_node(dom_id, parent_node)
+        if parent_node
+          [parent_node[:id], dom_id].join("-")
+        else
+          dom_id
+        end
+      end
+
       def engine
         renderer = @epub ? EpubRenderer : Renderer
         @engine ||= Redcarpet::Markdown.new(renderer,