Add cluster management pages to left nav

This MR is part of Docs project maintenance tasks, April 2023 (technical-writing#783 - closed), and adds four infrastructure pages to the left nav as identified in Pages not in the nav, Apr 2023 (#1616 - closed).

• Link to the merge request that introduced the new page:
• Give the MR a descriptive title.
• Optional. Link to the review app showing the new page: https://lc-docs-maintenance-april-nav2.docs.gitlab-review.app/ee/user/infrastructure/