[Add forge now] Create moderation guide

To help a moderator in the moderation process of a request we need proper documentation. This moderation guide should be accessible here: https://docs.softwareheritage.org/user/

Text draft for moderation guides is here: https://hedgedoc.softwareheritage.org/9z-HMpwsQnKx3jK41nnHKw?both#How-to-moderate-an-Add-Forge-request

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Migrated from T4037 (view on Phabricator)