Add translation hook for metadata content

[nbusseneau]

Problem: I wanted to localize the description metadata, but couldn't find a way to localize site.yaml.
I settled on using languages.yaml as a way to localize site.yaml metadata keys from metadata.html.twig, when they exist in both.

Users can override site.yaml metadata keys by providing a corresponding GRAV.METADATA.key via languages.yaml.

This is a working PR, but I don't believe this solution is elegant, even if it works fine. Input is welcome. Ideas?

[nbusseneau]

While skimming the doc, I discovered that we can actually override the description metadata at the page-level. Since pages are easily localized, localizing description for the home page is actually very easy. I am adding a fix to this translation hook so that page overrides take precedence when present.

Discussion: the translation hook trick would only be useful if one wants to localize the default global description tag for when one is not set up at the page level.

I have given it some more thought and I believe a good solution would be to mimic what's currently possible with templates: https://learn.getgrav.org/16/content/multi-language#language-based-twig-templates

Subfolders for overriding config values, e.g. config/fr/site.yaml.

What do you think?