Missing example custom dataset format or system outputs

[noelchen90]

We are missing documentation for examples in the following tasks + file types.
(Based on the file types that we do accept but are missing examples.)

• named-entity-recognition: system output - json
• chunking: system output- json
• word-segmentation: custom dataset - conll
• word-segmentation: system output - json, conll
• cloze-multiple-choice: custom dataset - json
• cloze-multiple-choice: system output - json
• cloze generative: custom dataset - json
• cloze-generative: system output - json
• aspect-based-sentiment-classification: custom dataset - json
• grammatical-error-correction: custom dataset - json
• grammatical-error-correction: system output - json
• language-modeling: custom dataset - json, text
• language-modeling: system output - json, text
• tabular classification: custom dataset - json
• tabular classification: system output - json, text

[noelchen90]

This is related to issue #300