Move and update name/location of vault page

Add navigation item to global navigation

Related to gitlab#471688

• Link to the merge request that introduced the new page: gitlab!165135 (merged)
• Give the MR a descriptive title.
• Optional. Link to the review app showing the new page: https://docs-update-secrets-nav.docs.gitlab-review.app/ee/ci/secrets/hashicorp_vault.html