Move pytest_plugins to top-level conftest.py for pytest 8.4+ compatibility

[Copilot]

Pytest 8.4+ no longer allows pytest_plugins in non-top-level conftest files. This was causing test collection to fail with:

Defining 'pytest_plugins' in a non-top-level conftest is no longer supported:
It affects the entire test suite instead of just below the conftest as expected.
  /home/runner/work/readthedocs.org/readthedocs.org/readthedocs/conftest.py

Changes

• Created /conftest.py with the pytest_plugins = ("sphinx.testing.fixtures",) declaration
• Removed pytest_plugins from readthedocs/conftest.py, preserving all fixtures

All 2459 tests now collect and run successfully.

Original prompt

Move pytest_plugins to top-level conftest.py for pytest compatibility

This PR moves the pytest_plugins assignment from readthedocs/conftest.py to a new top-level conftest.py file, as required by recent pytest versions. This resolves the error:

Defining 'pytest_plugins' in a non-top-level conftest is no longer supported

Now, all tests can be collected and run as expected with modern pytest.

No functional changes to tests or fixtures; this is a compatibility fix only.

Created from VS Code via the GitHub Pull Request extension.

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