Document how to configure a Riseup account in Claws

That’s quite popular and would be nice to be documented upstream: https://help.riseup.net/en/email/clients

Feature Branch: bitingbird:doc/8301-riseup_claws

Original created by @sajolida on 8301 (Redmine)

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information