[hotfix] accomodate and document re-registration flow

context

• Sometimes the signald keystore for a channel's phone number gets in a borked state and the only way to recover is to delete the keystore and re-register the phone number in order to force signald to rebuild the keystore from scratch.

changes

• add a playbook for re-registering phone numbers to the /docs dir
• increase the signald request timeout from 20s to 30s (which is necessary to accomodate the re-registration process)