DOC: Fix capitalization among headings in documentation files (#32550)
Headers updated for the following files:

```
- [x] doc/source/user_guide/options.rst
- [x] doc/source/user_guide/reshaping.rst
- [x] doc/source/user_guide/text.rst
- [x] doc/source/user_guide/timeseries.rst
- [x] doc/source/user_guide/visualization.rst
```

Other files updated in #32944, #32978, #32991  and #33147
