Use H2/H3 instead of H3/H4 respectively

General CSS hygiene dictates that you shouldn't go beyond an H3 tag. H1
for the document title, H2 for section headings, and H3 for low-level
headings.

spec/classes/paragraph_converter_spec.cr

@@ -300,8 +300,8 @@ describe ParagraphConverter do
       ]
     JSON
     expected = [
+      Heading2.new([Text.new("text")] of Child),
       Heading3.new([Text.new("text")] of Child),
-      Heading4.new([Text.new("text")] of Child),
       Paragraph.new([Text.new("text")] of Child),
       Preformatted.new([Text.new("text")] of Child),
       BlockQuote.new([Text.new("text")] of Child), # BQ

spec/components/page_content_spec.cr

@@ -109,26 +109,26 @@ describe PageContent do
 
   it "renders an H3" do
     page = Page.new(nodes: [
-      Heading3.new(children: [
+      Heading2.new(children: [
         Text.new(content: "Title!"),
       ] of Child),
     ] of Child)
 
     html = PageContent.new(page: page).render_to_string
 
-    html.should eq %(<h3>Title!</h3>)
+    html.should eq %(<h2>Title!</h2>)
   end
 
   it "renders an H4" do
     page = Page.new(nodes: [
-      Heading4.new(children: [
+      Heading3.new(children: [
         Text.new(content: "In Conclusion..."),
       ] of Child),
     ] of Child)
 
     html = PageContent.new(page: page).render_to_string
 
-    html.should eq %(<h4>In Conclusion...</h4>)
+    html.should eq %(<h3>In Conclusion...</h3>)
   end
 
   it "renders an image" do

src/classes/paragraph_converter.cr

@@ -13,11 +13,11 @@ class ParagraphConverter
       when PostResponse::ParagraphType::H3
         paragraph = paragraphs.shift
         children = MarkupConverter.convert(paragraph.text, paragraph.markups)
-        node = Heading3.new(children: children)
+        node = Heading2.new(children: children)
       when PostResponse::ParagraphType::H4
         paragraph = paragraphs.shift
         children = MarkupConverter.convert(paragraph.text, paragraph.markups)
-        node = Heading4.new(children: children)
+        node = Heading3.new(children: children)
       when PostResponse::ParagraphType::IFRAME
         paragraph = paragraphs.shift
         if iframe = paragraph.iframe

src/components/page_content.cr

@@ -46,12 +46,12 @@ class PageContent < BaseComponent
     figcaption { render_children(node.children) }
   end
 
-  def render_child(node : Heading3)
-    h3 { render_children(node.children) }
+  def render_child(node : Heading2)
+    h2 { render_children(node.children) }
   end
 
-  def render_child(node : Heading4)
-    h4 { render_children(node.children) }
+  def render_child(node : Heading3)
+    h3 { render_children(node.children) }
   end
 
   def render_child(child : IFrame)

src/models/nodes.cr

@@ -39,10 +39,10 @@ module Nodes
   class FigureCaption < Container
   end
 
-  class Heading3 < Container
+  class Heading2 < Container
   end
 
-  class Heading4 < Container
+  class Heading3 < Container
   end
 
   class ListItem < Container