refactor: [M3-7578] - Placeholder and EntityDetails v7 storybook migrations

[coliu-akamai]

Description 📝

This is the last of the storybook migrations!! (excluding intro.stories.mdx which is here to stay due to being a docs only file)

• Storybook migrations for Placeholder and Entity Details
• added tests as well

How to test 🧪

Verification steps

• http://localhost:6006/?path=/docs/features-entity-landing-page-placeholders--documentation
• http://localhost:6006/?path=/docs/features-entity-detail--documentation
• yarn test EntityDetail.test.tsx
• yarn test Placeholder.test.tsx

As an Author I have considered 🤔

Check all that apply

• 👀 Doing a self review
• ❔ Our contribution guidelines
• 🤏 Splitting feature into small PRs
• ➕ Adding a changeset
• 🧪 Providing/Improving test coverage
• 🔐 Removing all sensitive information from the code and PR description
• 🚩 Using a feature flag to protect the release
• 👣 Providing comprehensive reproduction steps
• 📑 Providing or updating our documentation
• 🕛 Scheduling a pair reviewing session
• 📱 Providing mobile support
• ♿ Providing accessibility support

[mjac0bs]

Test coverage looks good and Storybook stories render.

[github-actions]

Coverage Report: ✅
Base Coverage: 79.16%
Current Coverage: 79.24%