WIP: Refactor and restructure integration documentation

What does this MR do?

Refactor and move integration to administration/integration.

Are there points in the code the reviewer needs to double check?

• Links not borken.

Why was this MR needed?

One step closer to https://gitlab.com/gitlab-org/gitlab-ce/issues/3349.

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/21742.