Until now, the final copyright year on all the Gnuastro files was 2025. But
we are now in 2026, so it was necessary to update them. Also, the citation
statistics shown in the top Gnuastro webpage didn't include the full 2025.

With this commit, all the copyright years have been updated and the
citations sections of the webpage has been updated to be more clear
(including citation of the book). Also, the webpage didn't follow our
general documentation format of one sentence per line and that was also
corrected.