[Add forge now] Document new feature

The documentation of the new feature is important to create in parallel of the new feature. Here we need to add entry points in the documentation that will provide enough information for the different types of personas:

• visitors (on WordPress)
• users of submission form (on https://docs.softwareheritage.org/user/)
• users of interface as moderators (also on https://docs.softwareheritage.org/user/)
• contributors (https://docs.softwareheritage.org/devel)
• contributors on the sys-admin side, for example:
  • what's the process to add a forge to the scheduler after it was accepted (since it is a semi manual process)
  • on https://docs.softwareheritage.org/sysadm/

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Migrated from T4038 (view on Phabricator)