Add Information About Donating to BetterWorldBooks

I am pretty sure I heard in an Open Library zoom presentation that one could send books to that BetterWorldBooks as a way to donate to Open Library. I cannot find information about this on the Open Library website.

### Describe the problem that you'd like solved
Information about how donating to BetterWorldBooks is linked to Open Library is hard to find / does not exist on the Open Library website.

### Proposal & Constraints
- Add information about donating to BetterWorldBooks on the Sponsor a Book or Book Drive page.
- Include comparable differences between donating directly to Open Library vs. to BetterWorldBooks so user can make the best decision for how to donate.

### Additional context
Steps to Recreate:
1. I clicked More > Sponsor A Book
2. All the way at the bottom is a section called "Can I donate a physical book?" and I clicked the link there.
3. This takes me to the bookdrive page. I can not find info on this page about BetterWorldBooks.

### Stakeholders
@mekarpeles