CLN: Rename "add" to "sum" in groupby
- [ ] closes #xxxx (Replace xxxx with the Github issue number)
- [ ] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [x] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [ ] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

sum is the one method whose name doesn't agree with the corresponding `how`
When looking at this it reminded me of Pokemon: 'are you sure you caught them all?' @attack68 - I think our test coverage is sufficient to feel good I got all the code changes. For comments, I would guess it's possible I missed some. I used `grep -nr --include \*.p* "add"` in both the groupby and the _libs directories and went through the results (and just double checked now). Thanks @rhshadrach 