Removed/updated outdated links to example system outputs

[PaulCCCCCCH]

A follow-up for #300 Since we have one complete doc for each task (with both the system output format and the custom dataset format), I have changed the tooltip text and leave it under the task field.

For tasks that currently have no doc, I provide links to an example system output and custom dataset whenever possible.

We don't have links for the following tasks yet:

• word-segmentation
• cloze-multiple-choice
• cloze-generative
• grammatical-error-correction
• language-modeling
• tabular-classification
• tabular-regression