docs: add a proposal for multi-schema change (#34037)
docs: add a proposal for multi-schema change (#34037)

ref pingcap/tidb#14766
pingcap/tidb,kyle-wang/tidb,pingcap/tidb,ngaut/tidb,ngaut/tidb,pingcap/tidb,pingcap/tidb,pingcap/tidb,ngaut/tidb,kyle-wang/tidb,kyle-wang/tidb,kyle-wang/tidb,ngaut/tidb,pingcap/tidb,kyle-wang/tidb,ngaut/tidb,kyle-wang/tidb,ngaut/tidb