workflows: Only publish doxygen html directory to gh-pages

This strips the html directory from the gh-pages output, so the docs are
accessible at https://stm32duino.github.io/STM32LoRaWAN/ now.

Author: matthijskooijman

.github/workflows/publish-docs.yml

@@ -12,4 +12,4 @@ jobs:
       - uses: DenverCoder1/doxygen-github-pages-action@v1.1.0
         with:
           github_token: ${{ secrets.GITHUB_TOKEN }}
-          folder: api-docs
+          folder: api-docs/html