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

@dsaxton, I combed through all of `doc/source/../*.rst` which is what #32316 was referring to. I'm fairly certain this catches all remaining references. > Interested in writing a doc linting rule such that we catch future usages of `Pandas`, etc?

I can take some time to look into doing that.
 Thanks again @LeviMatus, feel free to open a PR with the linting check as a follow-up