CI: temporarily pin numpydoc<1.7 to unblock docstring validation (GH#61720)

[evgmosme]

Temporarily pin numpydoc<1.7 to unblock the docstring-validation job.

numpydoc 1.7.0 raises
AttributeError: 'getset_descriptor' object has no attribute '__module__'
inside numpydoc/validate.py, causing pandas’ Code Checks / Docstring validation
step to fail before any pandas code is run (see GH #61720).

This PR

• pins numpydoc<1.7 in environment.yml
  (propagated to requirements-dev.txt);
• fixes a duplicate Returns / Yields section in
  pandas/_config/config.py::option_context;
• marks an expected warning in doc/user_guide/timeseries.rst
  with :okwarning: so Sphinx no longer treats it as an error.

Together these changes restore a green CI across all jobs.

Notes

• All changes are limited to dev/CI and docs—no impact on end users.
• Once an upstream fix lands in numpydoc, we’ll remove the version pin.
• No new tests are required; a successful CI run itself demonstrates the fix.

---

• closes BUG: CI docstring-validation fails with AttributeError in numpydoc validate.py #61720
• CI green locally (pre-commit run --all-files & doc/make.py --warnings-are-errors)
• added to doc/source/whatsnew/vX.X.X.rst → not needed (CI-only)

[evgmosme]

Superseded by #61725