DOC: Removing tailing whitespaces in .rst files
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

In #24216 we will start linting formatting errors in `.rst` files. Removing all the tailing whitespaces here, so we don't mix the cleaning with the code in that PR.