docs: introduce deprecation docs + app-theme deprecation

Signed-off-by: Patrik Oldsberg <poldsberg@gmail.com>

packages/core-app-api/src/app/AppThemeProvider.tsx

@@ -98,7 +98,7 @@ export function AppThemeProvider({ children }: PropsWithChildren<{}>) {
   console.warn(
     "DEPRECATION WARNING: A provided app theme is using the deprecated 'theme' property " +
       'and should be migrated to use a Provider instead. ' +
-      'See https://backstage.io/docs/deprecations/TODO for more info.',
+      'See https://backstage.io/docs/api/deprecations#app-theme for more info.',
   );
 
   return (

packages/core-plugin-api/src/apis/definitions/AppThemeApi.ts

@@ -42,7 +42,7 @@ export type AppTheme = {
 
   /**
    * The specialized MaterialUI theme instance.
-   * @deprecated use Provider instead, see https://backstage.io/docs/deprecations/TODO
+   * @deprecated use Provider instead, see https://backstage.io/docs/api/deprecations#app-theme
    */
   theme: BackstageTheme;