bug#54794: Clarify licence of guix.gnu.org screenshots

[Tobias Geerinckx-Rice]

Guix,

Devin Ulibarri from the FSF rightfully pointed out that the licence of, 
e.g., <https://guix.gnu.org/en/screenshots/virtual-machine/> is not 
obvious.  It should be made so on the Web page itself.

From memory I'd say these are ‘public domain’, and [0] concurs, but I 
doubt [0] was explicitly intended to cover these images not contributed 
by Luis Felipe.

My Web mail can't find it, but there's an ongoing thread about 
relicencing the blog posts to CC-(BY?)-SA-whatever 4.0.  If it applies 
well to images (and I think it does), that or CC0 seem like the two 
obvious choices.

Kind regards,

T G-R

Sent from a Web browser.  Excuse or enjoy my brevity.

[0]: 
https://git.savannah.gnu.org/cgit/guix/guix-artwork.git/tree/website/README#n101 
— last section.