make-doc: use the docs-assets target instead of the full-sphinx-build docs

so we do not build each swh-* twice with unavaoidable sphinx wrnings when building each repo locally.

also add an hidden toctree entry to ensure swh-*/index.rst are properly referenced

and thus prevent sphinx from complaining about rst files not being included in a toctree without having to insert :orphan: everywhere.

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Migrated from D650 (view on Phabricator)