STYLE, CI move validate_rst_title_capitalization check to pre-commit
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them

xref #39624 in which it took ~15 minutes for @rhshadrach to be told

```
Heading capitalization formatted incorrectly. Please correctly capitalize "pytest.xfail" to "Pytest.xfail" 
```

, like this such feedback will be provided much faster
thanks @MarcoGorelli 