Remove deprecated RTD workflow

We should instead install the GitHub App. (I don't have permissions to do this.) See the announcement: <https://about.readthedocs.com/blog/2025/06/announcing-our-github-app-beta/>

Remove deprecated RTD workflow
112b3252 · Ben Mares · Ricardo Vieira · 60ba7c71 · 60ba7c71
--- a/.github/workflows/rtd-link-preview.yml
+++ b/.github/workflows/rtd-link-preview.yml
-name: Read the Docs Pull Request Preview
-on:
-  # See <https://github.com/readthedocs/actions/issues/45>
-  pull_request_target:  # zizmor: ignore[dangerous-triggers]
-    types:
-      - opened
-jobs:
-  documentation-links:
-    runs-on: ubuntu-latest
-    permissions:
-      pull-requests: write
-    steps:
-      - uses: readthedocs/actions/preview@v1
-        with:
-          project-slug: "pytensor"