Add diff() and round() methods for Index and a test for each
- [x] closes #19708
- [x] [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).
- [ ] 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.

Forgot to pass parameters periods and decimals in the call of the functions, i will fix this. What should i do about the conflict? The conflict should be resolved without removing someone else's contribution  Sorry for the mess, i pulled all the changes to fix the conflict. If you like i can open a new PR. I think all you need to do is

```
git fetch upstream
git merge upstream/main
```

into this branch Thanks @GrammatikakisDimitris 