fix(v2): never remove trailing slash from site root like '/baseUrl/'

[slorber]

Motivation

Fixes #5077

With baseUrl = /xyz/ and trailingSlash: false, we should never transform the root route to /xyz

Otherwise the output folder will contain website/build/xyz.html instead of website/build/index.html

Trailing slash config should only apply to subroutes of the site, not the root / baseUrl / homepage of the site, or it can lead to 404 server errors

Have you read the Contributing Guidelines on pull requests?

yes

Test Plan

tests + dogfooding on our i18n sites (using locale baseurl)

[netlify]

✔️ [V2]

🔨 Explore the source changes: abea2ad

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/60daf9140325540008ebe8d4

😎 Browse the preview: https://deploy-preview-5082--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	73
🟢 Accessibility	98
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-5082--docusaurus-2.netlify.app/

[github-actions]

Size Change: 0 B

Total Size: 601 kB

ℹ️ View Unchanged

Filename	Size
website/build/assets/css/styles.********.css	87.7 kB
website/build/assets/js/main.********.js	415 kB
website/build/blog/2017/12/14/introducing-docusaurus/index.html	66.5 kB
website/build/docs/introduction/index.html	235 B
website/build/index.html	31.3 kB

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[netlify]

✔️ [V2]

🔨 Explore the source changes: fb42df8

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/60db1b59a879240007da2609

😎 Browse the preview: https://deploy-preview-5082--docusaurus-2.netlify.app