need a document to describe the states of table/op etc.

https://github.com/4paradigm/OpenMLDB/blob/main/src/proto/tablet.proto
This file defines the states, which will be shown in some commands, e.g., gettablestatus, showopstatus, but we do not have a doc to describe the definition of those states. Maybe we can add a new doc under the section of reference.

Examine src/proto/tablet.proto to understand the state definitions. Look for existing reference documentation structure in the project. Create a new document describing each state's meaning and when it appears, likely under a 'reference' section. Verify by checking commands like gettablestatus and showopstatus output.

索引モデルが issue の本文から書いたものです。