Add DOCUMENTER_KEY to Docs workflow

[penelopeysm]

The DocsNav workflow wasn't being triggered by Documenter.jl pushes to the gh-pages branch, because when Documenter pushes with GITHUB_TOKEN it doesn't trigger other workflows. See https://docs.github.com/en/actions/security-for-github-actions/security-guides/automatic-token-authentication#using-the-github_token-in-a-workflow

When you use the repository's GITHUB_TOKEN to perform tasks, events triggered by the GITHUB_TOKEN, with the exception of workflow_dispatch and repository_dispatch, will not create a new workflow run. This prevents you from accidentally creating recursive workflow runs.

This adds the DOCUMENTER_SSH key as an alternative means of authentication. It's meant as a temporary fix; the proper fix is to factorise out the DocsNav workflow into an action and then stick it inside the Docs workflow itself.

[penelopeysm]

Docs workflow ran: https://github.com/TuringLang/SSMProblems.jl/actions/runs/10285736766

DocsNav workflow also ran: https://github.com/TuringLang/SSMProblems.jl/actions/runs/10286302987

[yebai]

Thanks @penelopeysm!

[penelopeysm]

Thanks for having a look @yebai! I'll just go ahead and do the same if I find any other repos with the same issue.