Update documentation and add schema details for Custom Dataset Preview

[ravi-kumar-pilla]

Description

We have introduced custom dataset preview for our users allowing them to implement preview method. However, the documentation needs to be updated with some details regarding the expected dict schema and also add some validation for the NewTypes (eg., TablePreview).

Context

#1847 (comment)

Possible Implementation

1. The return type of the preview function should match one of the following types

TablePreview = NewType("TablePreview", dict)
ImagePreview = NewType("ImagePreview", bytes)
PlotlyPreview = NewType("PlotlyPreview", dict)
JSONPreview = NewType("JSONPreview", dict) 

2. Kedro-Viz expects the dict should contain the below schema -

   TablePreview :

   preview={
       'index': number[], 
       'columns': string[], 
       'data': any[][]  // List[List[Any]] 
   }

   index - An array of 0 indexed integers representing nrows
   columns - An array of strings representing names of ncolumns
   data - A 2D array representing data for the TablePreview

   Example -

   Catalog -

   companies:
     type: pandas.CSVDataset
     filepath: ${_base_location}/01_raw/companies.csv
     metadata:
       kedro-viz:
         layer: raw
         preview_args:
           nrows: 5

   TablePreview value returned from preview() function -

   preview={'index': [0, 1, 2, 3, 4], 'columns': ['id', 'company_rating', 'company_location', 'total_fleet_count', 'iata_approved'], 'data': [[35029, '100%', 'Niue', 4.0, 'f'], [30292, '67%', 'Anguilla', 6.0, 'f'], [19032, '67%', 'Russian Federation', 4.0, 'f'], [8238, '91%', 'Barbados', 15.0, 't'], [30342, nan, 'Sao Tome and Principe', 2.0, 't']]}

3. We should enforce the schema in the NewType that is introduced to avoid blank UI as mentioned here

4. We should document the expected {key:value} pairs.

NOTE: This ticket needs to be updated with schema details of other return types (ImagePreview, PlotlyPreview, JSONPreview) for reference

Possible Alternatives

Checklist

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[Huongg]

to summarise: we're going to document the table preview and what schema exactly we're expecting.

[rashidakanchwala]

We need to also discuss how to be approach TablePreview when we do this ticket.

[astrojuanlu]

I was trying to prove that this should work for non-Pandas datasets but I couldn't get it to work at all #1847 (comment)