Add DOCUMENTER_KEY for tagbot.

Ref. "TagBot & tagged versions" under  https://juliadocs.github.io/Documenter.jl/stable/man/hosting/#GitHub-Actions. This is required for deploying docs for tagged versions and for the `stable` alias.

Author: ToucheSir

.github/workflows/TagBot.yml

@@ -10,3 +10,4 @@ jobs:
       - uses: JuliaRegistries/TagBot@v1
         with:
           token: ${{ secrets.GITHUB_TOKEN }}
+          ssh: ${{ secrets.DOCUMENTER_KEY }}