doc: remove files left over from pre-Antora doc builds - #88

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, actions/setup-node@v4, actions/upload-artifact@v4, alandefreitas/cpp-actions/boost-clone@v1.8.8, alandefreitas/cpp-actions/package-install@v1.8.8, seanmiddleditch/gha-setup-ninja@v5. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/

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