docs(VDataTable): add missing item column props

[kingyue737]

Description

Add missing index scoped prop of item column in VDataTable

Motivation and Context

Users can add a column of index by the following code:

<v-data-table :headers="[{ text: '#', value: 'index' }]"
  <template #item.index="{ index }">
    {{ index + 1 }}
  </template>
</v-data-table>

But index slot prop of item column does not exist in docs.

I used web-types of vuetify 2 to generate global component types for Volar intellisense and type check. Then I found index scoped prop of #item.<name> is missing in docs of VDataTable, resulting into type error.

How Has This Been Tested?

Markup:

// Paste your FULL Playground.vue here

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any features but makes things better)

Checklist:

• The PR title is no longer than 64 characters.
• The PR is submitted to the correct branch (master for bug fixes and documentation updates, dev for new features and backwards compatible changes and next for non-backwards compatible changes).
• My code follows the code style of this project.
• I've added relevant changes to the documentation (applies to new features and breaking changes in core library)