feat: support resource explanation rendering

Support resource explanation rendering of rspec api documentation.
See more details in zipmark/rspec_api_documentation#269 .

Author: yachun-ikala

app/views/apitome/docs/_all_examples.html.erb

@@ -1,6 +1,9 @@
 <% resources.each do |resource| %>
   <section class="example" id="<%= id_for(resource['name']) %>">
     <h1 class="resource-name"><%= resource['name'] %></h1>
+
+    <%= render partial: 'resource_explanation', locals: {explanation: resource['explanation']} if resource['explanation'] %>
+
     <% resource['examples'].each do |example| %>
       <% link = example['link'].gsub('.json', '') %>
       <article id="<%= id_for(link) %>">

app/views/apitome/docs/_resource_explanation.html.erb

@@ -0,0 +1,3 @@
+<article class="resource_explanation">
+  <%= raw rendered_markdown(explanation.strip_heredoc) %>
+</article>