Provide support for a footer extension point on the portal article page

JSD supports multiple footer extension points such as atl.footer and serviceDeskPortalFooter, but neither appear on the portal article page.

This suggestion relates to the community discussion in https://community.developer.atlassian.com/t/how-to-add-servicedeskportalfooters-or-similar-to-jsd-cloud-knowledgebase-articles/38797.