Put back doc deploy guard

Author: dstansby

Diff for: .github/workflows/docs.yml

@@ -46,6 +46,7 @@ jobs:
     name: Upload docs to GitHub pages
     runs-on: ubuntu-latest
     needs: build-docs
+    if: contains(github.ref, 'tags')
     steps:
     - uses: actions/checkout@v3
     - uses: actions/download-artifact@v3