Add `DOCUMENTER_KEY` secret so releases deploy documentation

The repository has no Actions secrets (gh api repos/JuliaDiff/ReverseDiff.jl/actions/secrets returns total_count: 0), so secrets.DOCUMENTER_KEY in TagBot.yml expands to an empty string.

TagBot therefore pushes release tags over HTTPS with GITHUB_TOKEN, and GitHub does not start workflow runs for refs pushed with GITHUB_TOKEN. The docs job in ci.yml never runs on a tag, so no versioned documentation has been deployed since v1.15.3 and stable still serves 1.15.3 (#267, #260).

ForwardDiff.jl has the same workflow files but does have the secret, and its tag pushes do trigger CI.

This needs someone with admin rights on the repository:

1. Run DocumenterTools.genkeys(user = "JuliaDiff", repo = "ReverseDiff.jl").
2. Add the public key under Settings → Deploy keys, with write access enabled.
3. Add the private key as a repository secret named DOCUMENTER_KEY.

Review TagBot.yml and ci.yml to confirm how DOCUMENTER_KEY is consumed and how tag pushes trigger the docs job. Run DocumenterTools.genkeys for JuliaDiff/ReverseDiff.jl, configure the deploy key and repository secret as described, then verify a release tag starts the docs workflow and deploys the versioned documentation.