DOC: normalize usage of word "pandas"
changes references to the library, pandas, to match the standard lowercase spelling. This changes applicable `.rst` files under the `doc/source` path.

References such as Pandas, **pandas**, *pandas*, and ``pandas`` have been replaced with pandas.

- [x] closes #32316 
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry
