Main documentation build broken by missing swh-web-specific setup

The swh.web module needs django initialized with a proper configuration enviroment variable before its documentation builds. This initialization is done in the swh-web specific docs config.

The main documentation build, which imports all subdocumentations, needs to also do this initialization. We probably need to have a generic pattern to do this initialization for all modules that need it.

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Migrated from T763 (view on Phabricator)