DOC: update multi-index term with MultiIndex
Hello,
before I start working on part 2 of the sphinx spelling extension I wanted to submit this PR in order to fix the double use of multi-index and MultiIndex. I have changed all `multi-index` mentions through the documentation to `MultiIndex`, this meant that some titles needed changing and new linking needed to be done. I have tested the documentation locally and everything seems to be working okay.

I have also reverted the changes to some adverbs that got changed previously (which shouldn't have according to @h-vetinari)

I hope that creating this PR before working on part 2 is okay. If you would rather me add this and the next one together let me know and I'll update the title and whatnot 😄 👍 


- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
