Add ssh key to TagBot action (#38)

So that tagging can trigger Documenter to produce documentation for that
tag/release.

Author: martinholters

.github/workflows/TagBot.yml

@@ -9,3 +9,4 @@ jobs:
       - uses: JuliaRegistries/TagBot@v1
         with:
           token: ${{ secrets.GITHUB_TOKEN }}
+          ssh: ${{ secrets.DOCUMENTER_KEY }}