blog: Improve RSS feed (#4544)

* blog: Point rss discovery to canonical URL

Update discovery link to the current canonical URL
for where the blog RSS feed lives. The redirect
is kept for compatibility.

* blog: Fix broken self URL and improve title

* Fix 404 from https://www.sitespeed.io/feed/blog.xml to `https://www.sitespeed.io/feed.xml` which can confuse feed readers by updating their link but then losing the subscription to an error.
* Shorten title and start with "Sitespeed.io" so that it sorts well alongside many other blogs in a feed reader under the default suggested name.

---------

Co-authored-by: Peter Hedenskog <peter@soulgalore.com>

docs/_layouts/default.html

@@ -41,7 +41,7 @@ layout: compress
 	<link rel="apple-touch-icon-precomposed" sizes="72x72" href="{{site.baseurl}}/img/ico/sitespeed.io-72.png">
 	<link rel="apple-touch-icon-precomposed" href="{{site.baseurl}}/img/ico/sitespeed.io-57.png">
 	<link rel="shortcut icon" href="{{site.baseurl}}/img/ico/sitespeed.io.ico">
-	<link type="application/atom+xml" href="https://www.sitespeed.io/feed/index.xml" rel="alternate" />
+	<link type="application/atom+xml" href="https://www.sitespeed.io/feed/blog.xml" rel="alternate" />
 
 	<style>
 		{% include css/default.css %}

docs/feed/blog.xml

@@ -3,10 +3,10 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <rss version="2.0" xmlns:atom="http://www.w3.org/2005/Atom">
   <channel>
-    <title>The sitespeed.io blog</title>
+    <title>Sitespeed.io Blog</title>
     <description>The latest news from the sitespeed.io world.</description>
     <link>https://www.sitespeed.io</link>
-    <atom:link href="{{ site.url }}/feed.xml" rel="self" type="application/rss+xml" />
+    <atom:link href="{{ site.url }}/feed/blog.xml" rel="self" type="application/rss+xml" />
     {% for post in site.posts limit:10 %}
     <item>
       <title>{{ post.title | xml_escape }}</title>