refactor(content-docs): use shelljs instead of execa

[Josh-Cena]

Motivation

Resolve #4455. Up to this point, the only thing that we should fix is removing execa in favor of shelljs. Now, we only have fs-extra for FS operations and shelljs for "actual shell operations". del is used in a forked plugin which I think we shouldn't touch, mkdirp isn't used anymore.

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Updated test cases a little bit; can also check preview that updated data is rendered correctly

[netlify]

✔️ [V2]

🔨 Explore the source changes: f2455ff

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/6188d0e122bc1e0007373368

😎 Browse the preview: https://deploy-preview-5904--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	78
🟢 Accessibility	98
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-5904--docusaurus-2.netlify.app/

[github-actions]

Size Change: -3 B (0%)

Total Size: 881 kB

ℹ️ View Unchanged

Filename	Size	Change
website/.docusaurus/globalData.json	38.2 kB	0 B
website/build/assets/css/styles.********.css	94 kB	0 B
website/build/assets/js/main.********.js	474 kB	0 B
website/build/blog/2017/12/14/introducing-docusaurus/index.html	66.1 kB	0 B
website/build/blog/index.html	36.8 kB	0 B
website/build/docs/index.html	44 kB	0 B
website/build/docs/installation/index.html	51.5 kB	-3 B (0%)
website/build/index.html	29.5 kB	0 B
website/build/tests/docs/index.html	25.2 kB	0 B
website/build/tests/docs/standalone/index.html	21.7 kB	0 B

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