Use Read the Docs action v1 (#31)

Read the Docs repository was renamed from `readthedocs/readthedocs-preview` to `readthedocs/actions/`. Now, the `preview` action is under `readthedocs/actions/preview` and is tagged as `v1`

Author: humitos

.github/workflows/rtd-link-preview.yml

@@ -11,6 +11,6 @@ jobs:
   documentation-links:
     runs-on: ubuntu-latest
     steps:
-      - uses: readthedocs/readthedocs-preview@main
+      - uses: readthedocs/actions/preview@v1
         with:
           project-slug: "xarray-einstats"