The Integrations page lists all the integrations that can be configured,

acting as the parent for those pages. This change adds some convenient

references to the index of the administration guide that link to the

specific integrations. This helps both to advertise the integrations

available when making a cursory glance through the administration page

and to make it a bit easier to locate the docs for those already

familiar with integrations. These are shown as nested under the link to

the Integrations page.

Built the docs and tested the links.