Fix documentation build with Pluggy 1.7

[Lothnic]

Fixes #15190.

With Pluggy 1.7.0, tox -e docs fails because the inherited PluginManager.list_plugin_distinfo docstring links to the undocumented private class pluggy._compat.DistFacade. Sphinx cannot resolve that target, and the documentation build treats warnings as errors.

Add that exact class reference to the existing nitpick_ignore list under undocumented third-party types. Other missing references still fail the build. A contributor changelog entry records the fix.

Validation:

• Reproduced the failure on unmodified main, including a clean Sphinx build.
• tox -e docs passes with Python 3.14.6, Pluggy 1.7.0 and Sphinx 9.1.0.
• A clean strict build (sphinx-build -E -a -j 4 -W --keep-going -b html) also passes.
• Ruff checks, Ruff formatting and git diff --check pass.
• Before the fix, tox -e py314 completed with 4,695 passed, 48 skipped, 17 xfailed and 4 xpassed. The change only affects documentation configuration.

The full pre-commit run could not complete because DNS resolution failed while fetching a hook dependency from GitHub.

AI assistance was used for investigation, implementation and preparation of this description.

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