DOC: Fix capitalization among headings in documentation files (#32550)
Files updated:

```
- [ ] doc/source/development/policies.rst
- [ ] doc/source/development/roadmap.rst
- [ ] doc/source/ecosystem.rst
- [ ] doc/source/getting_started/comparison/comparison_with_sas.rst
- [ ] doc/source/getting_started/comparison/comparison_with_sql.rst
```

more files updated in different PRs: #32944, #32991