Add schema documentation for Image Text, Pairs Multilabel, Speech Text, Speech to Speech, Text Multilabel

[muhsatrio]

Since there are several new schemas on https://github.com/IndoNLP/nusa-crowd/tree/master/nusacrowd/utils/schemas, I think need to update the docs also in https://github.com/IndoNLP/nusa-crowd/blob/master/task_schemas.md