Add atom:link rel="self" to RSS feed template (#8213)

The [W3C validator](https://validator.w3.org/feed/check.cgi) says:
> This feed is valid, but interoperability with the widest range of feed readers could be improved by implementing the following recommendations.

With the added line, this should be fixed.

docs/_tutorials/convert-site-to-jekyll.md

@@ -437,6 +437,7 @@ layout: null
     <channel>
         <title>{{ site.title }}</title>
         <link>{{ site.url }}</link>
+        <atom:link href="{{ page.url | prepend: site.url }}" rel="self" type="application/rss+xml" />
         <description>{{ site.description }}</description>
         <lastBuildDate>{{ site.time | date_to_rfc822 }}</lastBuildDate>
         {% for post in site.posts %}