[Doc][Core]Add <NameMapping> Documentation

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- [X] I had searched in the [issues](https://github.com/DTStack/chunjun/issues) and found no similar document issue.

### Description

When we configure the synchronization of the whole database, we need to configure the mapping relationship to correspond to the data name, table name, field name of sink, it's "namemapping".
NameMapping likes
```json
"nameMapping": {
"schemaMappings": {
"TIEZHU": "public"
},
"tableMappings": {
"TIEZHU": {
"TEST_TWO": "test_two",
"two": "table2"
}
},
"fieldMappings": {
"TIEZHU": {
"TEST_TWO": {
"ID": "id",
"_DATE": "_date_",
"_TIMESTAMP": "_timestamp_",
"_STR": "_str_"
},
"two": {
"id": "id",
"name": "username"
}
}
}
}
```

### Documentation Links

_No response_

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