docs: Fix sphinx warning

the tutorial.md suggestion change was fishy and not working [1]

17:21:21  /var/lib/jenkins/workspace/DDOC/build-on-diff/swh-docs/docs/swh-fuse/tutorial.md:85: WARNING: hardcoded link 'https://archive.softwareheritage.org/api/' could be replaced by an extlink (try using ':swh_web:`api/`' instead)

So the tutorial got converted into markdown to nail it.

• [1] https://jenkins.softwareheritage.org/job/DDOC/job/build-on-diff/62/console

Related to swh-docs!294 (closed)

Test Plan

make -C docs happy now

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Migrated from D6970 (view on Phabricator)