docs/Makefile: Fix warning about file not being included in any toctree

Do not generate TOC with sphinx-apidoc.

Mark the apidoc/swh.rst file as orphan.

This should turn the doc dev build [1] on Jenkins green again once rebased on top of !205 (closed).

• [1] https://jenkins.softwareheritage.org/job/DDOC/job/dev/871/console

Depends on !205 (closed)

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Migrated from D7219 (view on Phabricator)