[docs] Explain how to use valueGetter to transform type

[alexfauquette]

Solves #1313

add a paragraph in "column types" section to explain that column of type "dateTime" should not be stored with a string value.

I added subsections to separate clearly the two topics:

• the additional properties
• data type

preview: https://deploy-preview-3003--material-ui-x.netlify.app/components/data-grid/columns/#column-types

[alexfauquette]

It seems that defining sortComparator is not mandatory when using valueGetter

because applySorting uses getSortCellParams to get the parameters to apply the sorting function. And in these params, the value is given by valueGetter if defined:

/~https://github.com/mui-org/material-ui-x/blob/2aa2bcf322cd6ea886b1c52ba1d7b6ac2cc3a0d9/packages/grid/_modules_/grid/hooks/features/rows/useGridParamsApi.ts#L132L155

I did some archeology, the comment has been added in the doc by this PR. And at that time, the applySorting was different.

Does someone see another reason why the documentation should ask dev to provide a sortComparator when using valueGetter here? Otherwise, I will remove it @mui-org/x