make pytest hide sphinx deprecation warnings

[AlexanderLanin]

pytest logs are spammed with sphinx warnings.

This is kind of a workaround, as adding a custom pytest.ini to docs-as-code would require duplicating all the content, which would be a horrible idea. I don't know if we need some pytest.ini merge ability? But that would be crazy as well.

[Copilot]

Pull Request Overview

This PR updates pytest configuration to suppress noisy third-party Sphinx-related deprecation warnings in test logs, improving signal-to-noise without duplicating config across projects.

• Convert filterwarnings to a multiline block and add targeted ignore rules for sphinx_needs/docutils deprecations
• Retain existing suppression for pytest.PytestExperimentalApiWarning

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