Doc: Consensus key

The consensus key is introduced in proto L. Release notes already have details about the new operations and rpc commands:

https://tezos.gitlab.io/protocols/alpha.html#consensus-key

This MR introduces more genric documentation to help delegates use the feature. This belongs to the "Key management" section.

Closes #3775 (closed).

Rendered doc can be found at: https://gitlab.com/tezos/tezos/-/blob/bdb3edddae08c1603d2a5b933f6d5aac5f48dfe3/docs/user/key-management.rst

This should be merged later, when protocol L is voted in.