DOC: Fix title capitalization in documentation files (#32550)
Corrected title capitalization in various .rst files to match the standard of capitalizing only the first word, unless a term like DataFrame or Series is involved.

Ran the  script to find and
correct heading issues in the following files:
- doc/source/user_guide/timedeltas.rst
- doc/source/whatsnew/v0.7.0.rst
- doc/source/whatsnew/v0.23.4.rst
- (… and so on)

Fixes part of issue #32550.

- [ ] closes #32550 

Thanks @steeleelliott03 