docs: Fix redondant extlink link warnings on archive.s.o
This fixes build [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)
Related to T3829
Migrated from D6967 (view on Phabricator)