Are any charities exempt from GDPR?
Only if you do not collect, store or process any personal data of any natural person. This is very unlikely.
Do charities need to register with the ICO?
If your charity collects, stores and processes personal data then yes your charity will need to register with the ICO.
Do charities need to have documentation for GDPR?
Yes, charities are no different to a business and need to have the relevant documentation in place to ensure compliance.