[Courses] "Coming Soon" Styles

[una]

Changes in this PR:

• Apply differentiated "coming soon" styles to placeholder content for courses in sidebar
• Update partials to not render links to placeholder content
• Adds placeholder: true to Learn Design courses

If we're happy with this solution, I can replicate on Learn PWA

---

[chrome-devrel-review-bot]

Hello! This is an automated review by our custom reviewbot. It updates automatically when code or GitHub comments in this pull request are created or updated.

Requested changes

If there are any common problems with the content files you created or modified, they will be listed here.

src/site/content/en/learn/design/accessibility/index.md

• This file passed all of our automated Markdown audits.

src/site/content/en/learn/design/icons/index.md

• This file passed all of our automated Markdown audits.

src/site/content/en/learn/design/interaction/index.md

• This file passed all of our automated Markdown audits.

src/site/content/en/learn/design/media-features/index.md

• This file passed all of our automated Markdown audits.

src/site/content/en/learn/design/picture-element/index.md

• This file passed all of our automated Markdown audits.

src/site/content/en/learn/design/responsive-images/index.md

• This file passed all of our automated Markdown audits.

src/site/content/en/learn/design/screen-configurations/index.md

• This file passed all of our automated Markdown audits.

src/site/content/en/learn/design/theming/index.md

• This file passed all of our automated Markdown audits.

src/site/content/en/learn/design/typography/index.md

• This file passed all of our automated Markdown audits.

src/site/content/en/learn/design/ui-patterns/index.md

• This file passed all of our automated Markdown audits.

src/site/content/en/learn/pwa/app-design/index.md

• This file passed all of our automated Markdown audits.

src/site/content/en/learn/pwa/architecture/index.md

• This file passed all of our automated Markdown audits.

src/site/content/en/learn/pwa/assets-and-data/index.md

• This file passed all of our automated Markdown audits.

src/site/content/en/learn/pwa/caching/index.md

• This file passed all of our automated Markdown audits.

src/site/content/en/learn/pwa/capabilities/index.md

• This file passed all of our automated Markdown audits.

src/site/content/en/learn/pwa/complexity/index.md

• This file passed all of our automated Markdown audits.

src/site/content/en/learn/pwa/conclusion/index.md

• This file passed all of our automated Markdown audits.

src/site/content/en/learn/pwa/detection/index.md

• This file passed all of our automated Markdown audits.

src/site/content/en/learn/pwa/enhancements/index.md

• This file passed all of our automated Markdown audits.

src/site/content/en/learn/pwa/experimental/index.md

• This file passed all of our automated Markdown audits.

src/site/content/en/learn/pwa/installation-prompt/index.md

• This file passed all of our automated Markdown audits.

src/site/content/en/learn/pwa/installation/index.md

• This file passed all of our automated Markdown audits.

src/site/content/en/learn/pwa/offline-data/index.md

• This file passed all of our automated Markdown audits.

src/site/content/en/learn/pwa/os-integration/index.md

• This file passed all of our automated Markdown audits.

src/site/content/en/learn/pwa/service-workers/index.md

• This file passed all of our automated Markdown audits.

src/site/content/en/learn/pwa/serving/index.md

• This file passed all of our automated Markdown audits.

src/site/content/en/learn/pwa/tools-and-debug/index.md

• This file passed all of our automated Markdown audits.

src/site/content/en/learn/pwa/update/index.md

• This file passed all of our automated Markdown audits.

src/site/content/en/learn/pwa/web-app-manifest/index.md

• This file passed all of our automated Markdown audits.

src/site/content/en/learn/pwa/windows/index.md

• This file passed all of our automated Markdown audits.

src/site/content/en/learn/pwa/workbox/index.md

• This file passed all of our automated Markdown audits.

[netlify]

✔️ Deploy Preview for web-dev-staging ready!

🔨 Explore the source changes: d8dc9c8

🔍 Inspect the deploy log: https://app.netlify.com/sites/web-dev-staging/deploys/618175d9454b4000074235ac

😎 Browse the preview: https://deploy-preview-6649--web-dev-staging.netlify.app