infra: fix git push in the docs workflow

[maximlt]

I previously set up permissions in the pr_flow workflow which propagate to the workflow it calls. When a PR is merged, the docs workflow runs on its own, it needs additional permissions to push to the repo. Note that setting one permission disables all the other ones (see https://docs.github.com/en/actions/using-jobs/assigning-permissions-to-jobs), so I had to explicitly allow writing to pull-requests too. Let's see!

[github-actions]

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