add ssh key to docs.yml (#803)

Author: jishnub

.github/workflows/docs.yml

@@ -38,4 +38,5 @@ jobs:
       - name: Build and deploy
         env:
           GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          DOCUMENTER_KEY: ${{ secrets.DOCUMENTER_KEY }} # If authenticating with SSH deploy key
         run: julia --project=docs/ docs/make.jl