Add discoverability-action

[amrocha]

WHY are these changes introduced?

We don't have much observability into what the effect of our changes is, and sometimes that can cause unexpected bugs. This tries to address that.

WHAT is this pull request doing?

Adds a github action that runs yarn splash and comments on PRs with their splash zone

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

[github-actions]

No changes

💦 Potential splash zone of changes introduced to src/**/*.tsx in this pull request:

No significant changes to src/**/*.tsx were detected.

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