Remove 'Subpackages'/'Submodules'/'Module content' headings to clear the TOC

They are polluting the doc by doubling the depth of the hierarchy and provide no clear gain.

Also increase the depth of the generated TOC, so that we can see the full module hierarchy instead of just the first level.

Central apidoc index

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After

swh.storage documentation

change is only at the bottom of the page

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After

swh-storage apidoc

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After

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Migrated from D5261 (view on Phabricator)