Users happening across any of the Container pages may not already know the intent of Containers, or which others are available. This commit adds a link to the existing "GUI containers" page in their tutorials sections.
(cherry picked from commit 3b941ed98c)
This makes it easier to spot syntax errors when editing the
class reference. The schema is referenced locally so validation
can still work offline.
Each class XML's schema conformance is also checked on GitHub Actions.
