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

---

- [x] closes #61720  
- [x] 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)


Superseded by #61725