DOC/TST: Added NORMALIZE_WHITESPACE flag
I think we want this to be always on. You can disable it with

```
doctest: -NORMALIZE_WHITESPACE
```
In the validation script we also set `IGNORE_EXCEPTION_DETAIL`, and have this in https://github.com/pandas-dev/pandas/pull/19952 as well (but give that's still some time before that will be merged, fine to add it already in this PR) Added `IGNORE_EXCEPTION_DETAIL` so we're consistent.