OIDC + Slack Token for `nodejs/doc-kit`

Ref: https://github.com/nodejs/doc-kit/pull/624 `doc-kit` needs: - [x] A `SLACK_TOKEN` to publish alerts in Slack (cc @bensternthal) - [x] An OIDC token for ~~`@nodejs/doc-kit`~~ `@node-core/doc-kit` set up as follows: <img width="794" height="590" alt="Image" src="https://github.com/user-attachments/assets/1ff47dd3-c549-42d6-a659-c20b8c973e5a" /> cc @nodejs/TSC