Add FAQ for libraries that get removed

[allyhawkins]

Purpose

Here I'm adding a FAQ to explain why some libraries were previously there but might be missing. I tried to focus on the quality control and mentioned that we do not do any screening.

I'm also filing this to main based on our discussion in Slack. We don't need to release this immediately, but it will now be included in the next release. If we prefer to add it to development and then to the AnnData release branch, I can move it there.

Issue addressed

Closes #143

Checklist for publishing to main

• [x] This branch contains all commits/content ready to be released.
• [ ] The most recent CHANGELOG entry includes a date. If this release required a CHANGELOG entry, it should be today's date.

[allyhawkins]

I updated this to reflect your changes @jaclyn-taroni so should be ready for another look.