Added downgrade note in installation.mdx

Author: LucaVmu

docs/installation.mdx

@@ -10,6 +10,12 @@ import useBaseUrl from '@docusaurus/useBaseUrl';
 
 Remember Nyxian is mainly developed for jailed devices, so it's the recommended way of installing Nyxian.
 
+:::warning
+
+Do not downgrade Nyxian! You will lose all your data!
+
+:::
+
 <Tabs queryString="device">
    <TabItem value="jailed" label="Jailed" default>
       ## Jailed