[data grid] Add selectors to support showing child row count in footer

[KenanYusuf]

Users are looking to display the child row count when they have grouped rows.

Fixes #13638

• Adds two new selectors:

  {
    "name": "gridFilteredDescendantRowCountSelector",
    "returnType": "number",
    "category": "Filtering",
    "description": "Get the amount of descendant rows accessible after the filtering process.",
    "supportsApiRef": true
  },
  {
    "name": "gridFilteredRowCountSelector",
    "returnType": "number",
    "category": "Filtering",
    "description": "Get the amount of rows accessible after the filtering process.\nIncludes top level and descendant rows.",
    "supportsApiRef": true
  },

• Adds example of showing child row count in footer to the row grouping recipes https://deploy-preview-13725--material-ui-x.netlify.app/x/react-data-grid/recipes-row-grouping/#dispaying-child-row-count-in-footer

• I have followed (at least) the PR section of the contributing guide.

[mui-bot]

Deploy preview: https://deploy-preview-13725--material-ui-x.netlify.app/

Updated pages:

• docs/data/data-grid/recipes-row-grouping/recipes-row-grouping.md

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