LFDT-Lockness/cggmp21: docs/key_share.md

Key share is obtained as output of [key refresh protocol](mod@crate::key_refresh). It contains a [core share](IncompleteKeyShare) and auxiliary data required to carry out signing. Compared to the paper, we removed the El-Gamal private key as it's not used for 3-round presigning, which is the only on