BUG: new method for join with multiindex and single index (#34292)
- [x] closes #34292
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Tests still to be added.
I replaced the method to join a multiindex & a single index because outer joins didn't include data from the single index. The new method is similar to a join between 2 multiindexes.
I'm not sure what the keep_order parameter was, but it was never used and in other _join-methods it didn't exist either. So I suggest removing it all together.