ci: bump actions/upload-pages-artifact from 2 to 3 (#15)

Bumps [actions/upload-pages-artifact](https://github.com/actions/upload-pages-artifact) from 2 to 3.
- [Release notes](https://github.com/actions/upload-pages-artifact/releases)
- [Commits](https://github.com/actions/upload-pages-artifact/compare/v2...v3)

---
updated-dependencies:
- dependency-name: actions/upload-pages-artifact
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

.github/workflows/site.yml

@@ -14,7 +14,7 @@ jobs:
       - env:
           NIXPKGS_ALLOW_UNFREE: "1"
         run: nix develop .#docs --impure -c hugo -s ./docs/
-      - uses: actions/upload-pages-artifact@v2
+      - uses: actions/upload-pages-artifact@v3
         with:
           path: "docs/public"
           retention-days: 7