Feature/1713 amp renderer

[pkarw]

Related issues

#1713

Short description and why it's useful

@NataliaTepluhina please update the docs:
Google Advanced Mobile pages are available with Vue Storefront 1.6. By default, the Product and Category pages do support AMP. To get the AMP page You should use the following URL:
http://localhost:3000/c/hoodies-and-sweatshirts-24 -> http://localhost:3000/amp/c/hoodies-and-sweatshirts-24.
The discovery links are added as <link rel="amphtml" .... The landing pages then distribute the traffic to canonical Vue Storefront links to enable all the features to work (by definition AMP doesn't support any additional JavaScript).

Important notice: AMP renderer works ONLY IN PRODUCTION MODE as the Advanced Content Output Options - specifically index.html template switching works only in production. In development mode, there can be CSR/SSR issues so don't even test it in development mode :-)

AMPHTML templates are provided by the src/themes/default-amp special purpose theme which inherits some parts from default theme

Screenshots of visual changes before/after (if there are any)

Upgrade Notes and Changelog

• No upgrade steps required (100% backward compatibility)
• I've updated the Upgrade notes and Changelog on how to port existing VS sites with this new feature

Contribution and currently important rules acceptance

• I read and followed contribution rules
• I read the TypeScript Action Plan and adjusted my PR according to it
• I read about Vue Storefront Modules and I am aware that every new feature should be a module