feat: add rel="canonical" link tag

This is a neat SEO optimization: search engines will choose one URL as
the canonical version and crawl that, and all other URLs will be
considered duplicate URLs and crawled less often.

In our case all versions hosted from /ipns/blog.ipfs.io/* at public
gateways will be marked as duplicate and merged adding SEO juice to the
canonical URL.

More:
https://support.google.com/webmasters/answer/139066?hl=en

Author: lidel

layouts/partials/head.html

@@ -29,6 +29,7 @@
 <link rel="stylesheet" href="/highlight/github-gist.css" />
 <link rel="stylesheet" href="/asciinema/asciinema-player.css" />
 
+<link rel="canonical" href="{{ .Permalink }}" />
 <link rel="alternate feed" href="/index.xml" type="application/rss+xml" />
 
 <script src="/asciinema/asciinema-player.js"></script>