update docs to remove /alpha

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

docs/permissions/custom-rules.md

@@ -173,7 +173,7 @@ The api for providing custom rules may differ between plugins, but there should
 
    ```ts title="packages/backend/src/index.ts"
    // catalog plugin
-   backend.add(import('@backstage/plugin-catalog-backend/alpha'));
+   backend.add(import('@backstage/plugin-catalog-backend'));
    backend.add(
      import('@backstage/plugin-catalog-backend-module-scaffolder-entity-model'),
    );

docs/permissions/getting-started.md

@@ -34,7 +34,7 @@ To help validate the permission framework is setup we'll create a Test Permissio
 
    ```ts title="packages/backend/src/index.ts"
    // permission plugin
-   backend.add(import('@backstage/plugin-permission-backend/alpha'));
+   backend.add(import('@backstage/plugin-permission-backend'));
    /* highlight-remove-start */
    backend.add(
      import('@backstage/plugin-permission-backend-module-allow-all-policy'),
@@ -85,7 +85,7 @@ To help validate the permission framework is setup we'll create a Test Permissio
 
    ```ts title="packages/backend/src/index.ts"
    // permission plugin
-   backend.add(import('@backstage/plugin-permission-backend/alpha'));
+   backend.add(import('@backstage/plugin-permission-backend'));
    /* highlight-add-next-line */
    backend.add(import('./extensions/permissionsPolicyExtension'));
    ```

docs/permissions/plugin-authors/01-setup.md

@@ -59,7 +59,7 @@ The source code is available here:
    //...
    /* highlight-add-start */
    // Installing the permission plugin
-   backend.add(import('@backstage/plugin-permission-backend/alpha'));
+   backend.add(import('@backstage/plugin-permission-backend'));
    // Installing the allow all permission policy module
    backend.add(
      import('@backstage/plugin-permission-backend-module-allow-all-policy'),