Add stateful chart migration to navigation

Add navigation item to global navigation

Add https://docs.gitlab.com/charts/installation/bundled_chart_migration/ to navigation.

• Link to the merge request that introduced the new page: gitlab-org/charts/gitlab!4687 (merged)
• Give the MR a descriptive title.
• Optional. Link to the review app showing the new page:

Relates Document migration from bundled MinIO, Redis, a... (gitlab-org/charts/gitlab#6230 - closed)