Add a section in the FAQs about annotation cache

> Looks good to me overall. Can we have a section in the FAQs about this? I always have a hard time wrapping my head around it and there already is a section about custom cache that could be extended with this I think

_Originally posted by @FriederikeHanssen in https://github.com/nf-core/sarek/pull/936#pullrequestreview-1289038014_
            