Move howto use the api to users docs instance

The howto is complete and useful but hidden. The current link is: https://docs.softwareheritage.org/devel/getting-started/api.html should be in : https://docs.softwareheritage.org/user/ With the link: https://docs.softwareheritage.org/user/getting-started/api.html

Should we have a redirection from devel/getting-started/api.html to user/getting-started/api.html?

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Migrated from T3912 (view on Phabricator)