Add alert for PF3 mapper

Author: rvsia

packages/react-renderer-demo/src/pages/mappers/pf3-component-mapper.md

@@ -1,12 +1,17 @@
 import DocPage from '@docs/doc-page';
 import ComponentMapperBar from '@docs/component-mapper-bar';
+import Alert from '@material-ui/lab/Alert';
 
 <DocPage>
 
 # PatternFly 3 mapper
 
 <ComponentMapperBar prefix="pf3" href="https://www.patternfly.org/v3/" />
 
+<Alert severity="error">With the future release of version 3.0 we are going to exclude PatternFly 3 mapper from our main branch. PF3 mapper won't be a part of this documenation and it won't get any new updates. The mapper will be moved to a seperate branch, so specific updates/bufixes will be still possible, but not guaranteed. (Version 3 should not include any breaking changes, so the mapper should work fine.) Please consider migrating to a different mapper, as PF3 does not support React 17+.</Alert>
+
+<br />
+
 PatternFly 3 mapper provides components from [PatternFly 3 design system](https://www.patternfly.org/v3/).
 
 ## Installation