[DataGrid] Add "Custom columns" page to the docs

[joserodolfofreitas]

Summary

It's a recurrent pain point that users try to find some of our column implementations used in the demos and can't find them behind the data grid generator, also we have no "targetted" page where users can quickly find some good examples of custom columns and how to implement it. "Rendering cells" is under column definition.

We need to improve the discoverability and better showcase good examples of custom columns like the star-rate component, date picker and now also the sparklines.

Proposal

A new page under the Columns menu with:

• a showcase of star-rate component reviewing important APIs used (like renderCell, valueFormatter and etc.)
• a showcase of sparklines integration with any needed additional concepts
• a showcase of date pickers integration with any needed additional concepts
• a Data Grid demo that doesn't use the data generator and showcases exclusively custom columns (star rate, progress bar, link, currency, sparklines and etc).