fix: add missing checkout step to docs-pr CI job

dorny/paths-filter needs a git checkout to run git diff.
The checkout step was accidentally removed when rewriting
the docs-only skip logic.

Signed-off-by: Simon Davies <simongdavies@users.noreply.github.com>

Author: simongdavies

.github/workflows/pr-validate.yml

@@ -23,6 +23,7 @@ jobs:
     outputs:
       docs-only: ${{ steps.docs-only.outputs.result }}
     steps:
+      - uses: actions/checkout@v6
       - uses: dorny/paths-filter@v3
         id: changes
         with: