Add Integrations top-level navigation

I noticed this comment: gitlab!37985 (comment 415170569) about adding a recently-added integration to the global navigation and recalled we don't really have a great place for them.

This MR adds Gitpod and lots of other integration topics to the navigation for the first time.

The top-level of the new menu structure is at: http://eread-add-integrations-to-nav.178.62.207.141.nip.io/ee/integration/README.html.

A nested landing page is at: http://eread-add-integrations-to-nav.178.62.207.141.nip.io/ee/administration/auth/README.html.

I didn't add http://eread-add-integrations-to-nav.178.62.207.141.nip.io/ee/integration/shibboleth.html as it's long deprecated.