fetch.pl: get GNU standards documents from gnulib

fetch.pl: get GNU standards documents from gnulib

Pulling these files from gnulib, instead of getting them directly
from their official home, means that all the files fetched by fetch.pl
come from Savannah Git repositories, which is particularly important
right now as Savannah CVS repositories are currently inaccessible via
the web, see
https://lists.gnu.org/archive/html/savannah-hackers/2026-02/msg00024.html
et seq.

* fetch.pl (to_fetch): Get all gnustandards files from gnulib instead.
  (savannah_url): Remove special case for gnustandards.