Pin Intel OneAPI to 2025.3 in CI workflows for coverage and docs

[vlad-perevezentsev]

This PR proposes using OneAPI 2025.3 for build-sphinx and generate_coverage workflows to match the compiler version used to build dpctl packages since dpctl is waiting for the latest compiler to become available on conda-forge dpctl#2300

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• Have you checked performance impact of proposed changes?
• Have you added documentation for your changes, if necessary?
• Have you added your changes to the changelog?

[github-actions]

View rendered docs @ https://intelpython.github.io/dpnp/pull/2887/index.html

[coveralls]

Coverage is 78.43% — pin_coverage_docs_build into master. No base build found for master.