CMS tool should have injectable fragments

[MattMonasch]

Is your feature request related to a problem? Please describe.
On a couple pages, we have a WARNING segment detailing the caveats of publishing separate ARM and x86 artifacts and how to not mess it up. These are copied and that makes it so that both need to be updated if we want to change the copy.

Describe the solution you'd like
We should have a way to define a fragment, like the aforementioned WARNING, that can then be included on a page.

Describe alternatives you've considered
No alts considered

Additional context
Warning seen in this segment https://chromeos.dev/en/publish/google-play-games#separated-projects
I cannot find a second instance but I could've sworn one existed or that we wanted to add another instance of this.