docs(v2): replace diff codeblocks with line highlight

[Josh-Cena]

Motivation

Resolve #4698. There are some diff code blocks with only + and no -. In this case using line highlight equally makes sense, with the additional merits of:

1. Not generating "confusing code" when using copy button;
2. Leverages syntax highlight to make the code block more visually appealing.

For the rest of the code blocks that have - as well, I kept them as-is.

Have you read the Contributing Guidelines on pull requests?

Yep

[netlify]

✔️ [V2]
Built without sensitive environment variables

🔨 Explore the source changes: 51b0836

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/60c937d780cd5c00090cbe64

😎 Browse the preview: https://deploy-preview-4973--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	69
🟢 Accessibility	97
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-4973--docusaurus-2.netlify.app/

[slorber]

Thanks 👍