Docs: Create ADR for default content versioning

We need a way to communicate notable changes to the default content of a slot. This should cover changes that aren't breaking changes to the API of a slot (no changes to plugin props or location on page), so plugins would still work, but any plugins that rely on the default content might need to be tweaked to account for the default content changes.

A good example of this comes from https://github.com/openedx/frontend-app-learning/pull/1713/ where slots are moving from having something as default content to having nothing. Any plugins that replace the default content entirely will continue to work without any changes, but any plugins designed around wrapping the default content would instead be wrapping nothing.

Start with this issue and the linked frontend-app-learning PR #1713 to compare default-content changes with plugin API changes. Draft an ADR that defines how non-breaking default content changes are communicated, including the distinction between plugins that replace content and those that wrap it; it is done when that guidance is documented and reviewable.