Add tpl/sites

[regisphilibert]

This should be labeled as a wish...

I know we should really limit those global function and the wish was to stop at site and hugo.
But there's still one page method desperately needed in the layout tree or "returning" partilas and for which we'd rather avoid having to pass the page context all the way down the "tree".

This is .Sites which I (maybe just me) often to find the "default language" site when in another language. (.Sites.First)

[bep]

You can do ... site.Sites.First.

[github-actions]

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