Box UI Elements installation page doesn't properly list the maximum React version (the <19 part)

The documentation regarding installation off [UI Elements](content/guides/embed/ui-elements/installation.md) doesn't list the maximum React version (<19).

Since React 19 has been in stable release since 2024-12-05 ([source](https://react.dev/blog/2024/12/05/react-19)), it's important to let users know they may need to downgrade React to use this the npm package for this library (until that's no longer the case).

I believe a few small copy edits on the page I referenced above would be enough to alert users to the situation although there may be other pages that repeat the same info (and thus would need similar changes).

Beginne mit content/guides/embed/ui-elements/installation.md und überprüfe die vorhandenen Hinweise zur Installation von React und npm. Durchsuche die Dokumentation nach wiederholten Hinweisen zur React-Kompatibilität von UI Elements, aktualisiere die relevanten Formulierungen so, dass sie die unterstützte maximale Version nennen, und stelle sicher, dass alle zutreffenden Installationsseiten die Einschränkung konsistent beschreiben.

Vom Indexierungsmodell aus dem Issue-Text verfasst.