Adds the last set of Learn Design modules

[rachelandrew]

Fixes: #6414
Fixes: #6416
Fixes: #6417
Fixes: #6418

The last batch of RWD modules. To be merged 23 Dec as part of Designcember.

[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/conclusion/index.md

• This file passed all of our automated Markdown audits.

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

[netlify]

✔️ Deploy Preview for web-dev-staging ready!

🔨 Explore the source changes: 79cb523

🔍 Inspect the deploy log: https://app.netlify.com/sites/web-dev-staging/deploys/61b8e33af1e21e00080695b5

😎 Browse the preview: https://deploy-preview-7017--web-dev-staging.netlify.app