chore: add canonical URL to head template

[davlgd]

Fix #260

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[imfing]

@davlgd thank you

[mafendi]

I am sorry, but it is a good idea to provide the possibility to specify the canonical URL if any. This is useful to content writers referring to another resource as original.

On our website https://leipapa.com we use the following structure:

Added canonical to front matter and ...

{{ with .Params.canonical }}
  <link rel="canonical" href="{{ . }}" itemprop="url" />
{{ else }}
  <link rel="canonical" href="{{ .Permalink }}" itemprop="url" />
{{ end }}

[imfing]

I am sorry, but it is a good idea to provide the possibility to specify the canonical URL if any. This is useful to content writers referring to another resource as original.

On our website https://leipapa.com we use the following structure:

Added canonical to front matter and ...

{{ with .Params.canonical }}

  <link rel="canonical" href="{{ . }}" itemprop="url" />

{{ else }}

  <link rel="canonical" href="{{ .Permalink }}" itemprop="url" />

{{ end }}

@mafendi hey, can you create a PR for this?