Add flag for draft pages

[gwyneplaine]

WHY are these changes introduced?

Fixes #8311

WHAT is this pull request doing?

This PR has the following changes:

• Resource index layout page is hidden from nav
• Resource index layout page has a 'noindex' tag in the head to omit it from search engine indexing
• draft flag implemented for patterns, that renders a Coming Soon component instead of the pattern contents when enabled (and in production)
• draft flag added to Resource index layout page.

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