DOC: Fix capitalization among headings in doc/source/user_guide/ part…
… 1 (#32550)

- [ ] xref #32550 
- [ ] tests added / passed
- [x] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

This pull request is stale because it has been open for thirty days with no activity. Please update or respond to this comment if you're still interested in working on this. @ShyamDesai can you merge master to fix conflicts? This generally looks good. Thanks but going to close this PR as stale. Please ping us if you would like to keep working on this. 