Link to Fortran in action / Modern Fortran programming

[awvwgk]

The Fortran Fans organization is providing the Fortran in action and Modern Fortran programming pages.

We should in include a reference to this resources on the learn page, either globally for all languages or investigate a mechanism to include localized material only for the respective translation. Personally, I would prefer the former and include the language of the linked page in the description, this allows translators to adapt the URL as necessary should the linked resource be multilingual as well.

cc @zoziha