[Add forge now] Create user guide

Document user guide on How to add forge now on https://docs.softwareheritage.org/user/

• Add link from feature to user guide

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Migrated from T4007 (view on Phabricator)

added Documentation priority:Normal + 1 deleted label

changed title from [Documentation] Document user guide on How to add forge now to [Add forge now] Document user guide

marked this issue as related to #4037

marked this issue as related to swh/meta#4038 (moved)

changed title from [Add forge now] Document user guide to [Add forge now] Create user guide

changed the description

marked this issue as related to swh/meta#4039 (moved)

marked this issue as related to #4059

marked this issue as related to #4245