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)

• [1] https://jenkins.softwareheritage.org/job/DDOC/job/dev/628/console

Related to T3829

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Migrated from D6967 (view on Phabricator)