Clarifies the dev-dependencies -> group.dev.dependencies switch in docs (#6723)

As written, it seemed to prioritize the older section notation without
clarifying that the older syntax still works but shouldn't be used. This
reorder visually prioritizes the preferred syntax as of 1.2.x while
clearly explaining what the old syntax is during this transition period.

The best next step here would be clearly defining the version in which
dev-dependencies will disappear.

Co-authored-by: Mathieu Kniewallner <mathieu.kniewallner@gmail.com>