Make AMP pages discoverable

[SantosGuillamot]

This is needed for AMP feature. For full context please check out the final Implementation proposal.

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AMP pages need a <link rel="amphtml"> element, like this:

<link rel="amphtml" href="https://domain.com/some-post/amp/" />

And HTML pages need a <link rel="canonical"> element, like this:

<link rel="canonical" href="https://domain.com/some-post/" />

Right now, it can be added manually to the theme, but we should solve it from the @frontity/amp package.