Add canonical tags to all pages

[danielepolencic]

We should add canonical tags to all the pages.

Tasks:

1. using the same logic in the sitemap (or backstop.ts test), for each page, load the page, parse it as Hast and check if a <link rel=“canonical” href=“https://example.com/sample-page/” /> exists.
2. add canonical to all pages, but not for:

• 404
• redirects
• RSS feeds

Please note that the tool should work against a working website such as http://localhost:4000 or https://learnk8s.io