Support defining a common header for all pages under a given root

As a page author, I would like the ability to set some header content that renders uniformly across all descendant pages under a given root page, so that I can automatically and easily enforce a unified header for all such associated pages.

For example, if my team has a root page at /MyTeamRoot and creates many pages underneath it, I would like a way to set a header that always appears for pages starting with that root, e.g. /MyTeamRoot/Sub/Page.

While it is of course possible to handle this manually by adding e.g. include at the top of every descendant page, it would be useful to have a generic strategy for applying the same content to all descendants of a given space/page automatically.

For groups of users who are collaborating on some pages under a particular root, this kind of function would save the group significant effort and enable them to enforce a uniform header across all the group's pages. As it stands, a team who wants this kind of consistency has to be diligent when creating new pages, making sure they always include the same chunk of markup at the top every time.