WEB: Sort and fix link to under discussions PDEPs
The links to the under discussion PDEPs in https://pandas.pydata.org/about/roadmap.html point to the API page, not the html page, fixing. Also, sorting the list by title.
> I guess sorting by title is better than the current status, but wouldn't this place PDEP-10 before PDEP-9?

Yes, you're correct. I realized when I finished implementing, but I didn't want to implement a more complex sorting using regex here, since this was originally to fix the links, and also we can consider other options like using `PDEP-0001`... (we're doing that with the file names). I'd personally merge this first, so the links are working, but happy to have the discussion here too. thx @datapythonista 