Fix docs workflow root path resolution in generate-docs.sh

[Copilot]

The build-and-deploy docs job failed because scripts/generate-docs.sh resolved the project root one directory too high in GitHub Actions, so Package.swift was not found. This PR corrects root resolution to match the repository checkout layout used by the workflow.

• Root cause addressed

  • PROJECT_ROOT was derived from scripts/ using dirname(dirname(...)), which moved outside the repo root in CI.

• Change

  • Update PROJECT_ROOT to the direct parent of scripts/ so the script always runs from the repository root before patching/building docs artifacts.

SCRIPT_DIR=$( cd -- "$( dirname -- "${BASH_SOURCE[0]}" )" &> /dev/null && pwd )
PROJECT_ROOT="$(dirname "$SCRIPT_DIR")"
cd "$PROJECT_ROOT"

[mickael-menu]

Confirmed it works: https://github.com/readium/swift-toolkit/actions/runs/28242680816

[Copilot]

Pull request overview

This PR fixes the working-directory resolution in scripts/generate-docs.sh so the docs generation job runs from the repository root in GitHub Actions (ensuring Package.swift is found).

Changes:

• Update PROJECT_ROOT to be the direct parent of scripts/ (instead of two levels up).
• Ensure the script cds into the repository root before performing doc build steps.

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