use GITHUB_TOKEN for better security

see:
https://docs.github.com/en/actions/security-guides/automatic-token-authentication

Author: ukkopahis

.github/workflows/main.yml

@@ -15,4 +15,4 @@ jobs:
       - name: Deploy docs
         uses: mhausenblas/mkdocs-deploy-gh-pages@master
         env:
-          PERSONAL_TOKEN: ${{ secrets.PERSONAL_TOKEN }}
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}