Add logos from guix-artwork repository and use them for "M-x guix-about".
* images/guix-logo.svg: New file.
* images/guixsd-logo.svg: New file.
* images/README: New file.
* Makefile.am (dist_image_DATA): Add images.
* elisp/guix-build-config.el.in (guix-config-image-directory): New constant.
* elisp/guix-default-config.el (guix-config-image-directory): Likewise.
* elisp/guix-config.el (guix-image-directory): New variable.
* elisp/guix-utils.el (guix-guixsd?): New procedure.
* elisp/guix-about.el (guix-about-buffer-name)
(guix-about-specifications): New variables.
(guix-logo-file, guix-insert-logo, guix-about-revert)
(guix-about-insert-content): New procedures.
(guix-about): New command.
* doc/emacs-guix.texi (Introduction): Mention it.
Alex Kost