Move logs/logs.md to administration/logs.md

What does this MR do?

Move logs.md documentation under administration/ where it fits better.

Why was this MR needed?

For a better doc structure.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/3349

mentioned in issue gitlab-com/doc-gitlab-com#64 (closed)

Status changed to merged

mentioned in commit 0587fe72

mentioned in merge request !4623 (merged)

mentioned in commit e8ebc1b6

mentioned in issue #3349 (moved)

Mentioned in commit pfjason/gitlab-ce@e8ebc1b6