doc: add example for dynamic session-scoped parametrized fixtures

[sedat4ras]

Closes #10156

Summary

Adds a documentation example to doc/en/example/special.rst showing how to build session-scoped parametrized fixtures whose params depend on command-line arguments.

The problem: A plain @pytest.fixture definition with params cannot use CLI option values because params are evaluated at import time — before pytest_configure runs. This trips up many users (the workaround was buried in a 2017 issue comment).

The solution: Register a plugin class containing the fixture inside pytest_configure, where CLI options are already available.

def pytest_configure(config):
    server_list = [s.strip() for s in config.getoption("--servers").split(",")]

    class DynamicFixturePlugin:
        @pytest.fixture(scope="session", params=server_list)
        def server_hostname(self, request):
            return request.param

    config.pluginmanager.register(DynamicFixturePlugin(), "server-hostname-fixture")

The example was adapted from the working code provided by the issue author in #10156.

Checklist

• Include documentation when adding new features.
• Include new tests or update existing tests when applicable. — N/A (docs only)
• Allow maintainers to push and squash when merging my commits.
• Closes Document how to build dynamic session-scoped fixtures #10156
• Created changelog/10156.doc.rst

AI attribution: This PR was written with AI assistance (Claude). The changes have been reviewed for correctness against pytest's fixture and plugin documentation.