Add Table of Contents to top-level index.md content files

[yurm04]

WHY are these changes introduced?

Fixes #10367

Not all index.md files need to show the TOC, but there are a few that do so we're going to pass a new showTOC prop in the frontmatter to enable this

WHAT is this pull request doing?

• add TOC prop to .md frontmatter
• pass showTOC prop to catchAllTemplate

How to 🎩

🖥 Local development instructions
🗒 General tophatting guidelines
📄 Changelog guidelines

Copy-paste this code in playground/Playground.tsx:

import React from 'react';
import {Page} from '../src';

export function Playground() {
  return (
    <Page title="Playground">
      {/* Add the code you want to test in here */}
    </Page>
  );
}

🎩 checklist

• Tested on mobile
• Tested on multiple browsers
• Tested for accessibility
• Updated the component's README.md with documentation changes
• Tophatted documentation changes in the style guide