Add a docs FAQ section

[haileyschoelkopf]

There's a lot of content in our docs but not all of it is easily discoverable for new users. We get some questions asked in issues frequently, which mainly means we need to improve discoverability.

Adding a docs/faq.md which contains QnA for common footguns / pitfalls / errors and how to resolve would be very useful.

[haileyschoelkopf]

Some things for which an FAQ would be very very helpful:

• "how is evaluation {request_type} implemented"? --> link to our new paper's appendix describing these details (https://arxiv.org/abs/2405.14782)
• Common mistakes when running the library offline without access to HuggingFace
• TL;DR on using saved-locally datasets
• ...