Add canonical URL links to HTML documentation to prevent search engines serving old versions of docs

[lpil]

Search engines are surfing links to outdated versions of package documentation. This is a problem as it misleads users, especially for core libraries such as the stdlib.

If the documentation had canonical links in the head element then search engines would hopefully suggest the current version instead.

<link rel="canonical" href="https://hexdocs.pm/package_name/path/to/file.html" />

Add this link to docs when publishing.