Display content of blog index or blog-section index

[chalin]

• The main use case here is to support the rendering introductory text to the blog/section.
• Currently the content of blog index or blog-section index pages is ignored without warning -- that shouldn't happen!
• This is a feature that we've had for the Community index page since Docsy's inception, see https://github.com/google/docsy/blame/7c865d0a2e0a7011dd92216e8c4013a1c37ffd7e/layouts/community/list.html#L15-L17:

  <div class="td-content">
  {{ .Content }}
  </div>

Tasks

• Blog list layout refactoring #2155
• Blog list layout: display the page title and content #2157
• Blog list layout whitespace cleanup #2159