Document usage of Diátaxis framework for pages

[awvwgk]

The documentation follows the Diátaxis framework (https://diataxis.fr/). A short overview in the design section should be added to clarify the four sections the documentation is subdivided in.

[vmagnin]

The documentations of fpm, stdlib and gtk-fortran are now listed on the Diataxis page:
https://diataxis.fr/adoption/