Create document on how to perform downgrade

[ahrtr]

Link to the issue etcd-io/etcd#11716

Also refer to https://github.com/ahrtr/etcd-issues/blob/master/docs/what_is_new_in_etcd_3_6.md#downgrading

Both @siyuanfoundation @fuweid have more context on the downgrade feature.

[siyuanfoundation]

could follow the similar doc structures in https://etcd.io/docs/v3.5/downgrades/downgrade_3_5/, with the Downgrade Procedure different.

[siyuanfoundation]

I will draft the initial doc.
/assign