docs/cli: Align cli documentation toc setup

This changes the deprecated :show-nested: instruction to the :nested: one [1]

Tentatively trying to fix failing build on master [2] to fix the failing build on doc [3]

• [1] https://sphinx-click.readthedocs.io/en/latest/usage/#directive-click

• [2] https://jenkins.softwareheritage.org/job/DFUSE/job/tests/723/consolex

• [3] Related to swh-docs!209 (closed) swh-docs!210 (closed)

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Migrated from D7466 (view on Phabricator)