documentation sidebar: group swh.* module doc under a "API reference" heading

The sidebar of docs.s.o is growing very long and unwieldy. We should structure the information there a bit more than the status quo. An easy win is to group all swh.* under a top-level entry "API reference", which is common place in tech documentation. When expanded it will give access to all the current swh.* as sub-entries.

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Migrated from T3737 (view on Phabricator)