Add Documenter SSH key to TagBot

Without this, documentation on tags isn't built at all

Author: giordano

.github/workflows/TagBot.yml

@@ -11,4 +11,5 @@ jobs:
     steps:
       - uses: JuliaRegistries/TagBot@v1
         with:
+          ssh: ${{ secrets.DOCUMENTER_KEY }}
           token: ${{ secrets.GITHUB_TOKEN }}