Update Documentation.yml for deployment keys

Added DOCUMENTER_KEY for SSH deploy authentication.

Author: lkdvos

.github/workflows/Documentation.yml

@@ -35,4 +35,5 @@ jobs:
       - name: Build and deploy
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }} # For authentication with GitHub Actions token
+          DOCUMENTER_KEY: ${{ secrets.DOCUMENTER_KEY }} # For authentication with SSH deploy key
         run: julia --project=docs/ docs/make.jl