[Add forge now] Create Help text tab

Display a help text tab in the submission form (same layout as in save code now view). Text is available here: https://hedgedoc.softwareheritage.org/9z-HMpwsQnKx3jK41nnHKw?both#Help-text-on-tab

Migrated from T3988 (view on Phabricator)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information