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

```
- [x] doc/source/user_guide/categorical.rst
- [x] doc/source/user_guide/computation.rst
- [x] doc/source/user_guide/cookbook.rst
- [x] doc/source/user_guide/gotchas.rst
- [x] doc/source/user_guide/groupby.rst
```

Other files updated in #32944, #32978 , #32991 and #33149