docs: Fix redondant extlink link warnings on archive.s.o
This fixes warning like [1] and attempt to fix the build [2]
[1]
16:47:16 /var/lib/jenkins/workspace/DDOC/build-on-diff/swh-docs/user/faq/index.rst:41: WARNING: hardcoded link 'https://archive.softwareheritage.org/save/' could be replaced by an extlink (try using ':swh_web:`save/`' instead)
[2] https://jenkins.softwareheritage.org/job/DDOC/job/build-on-diff/59/console