fix(v2): fix minor a11y issues

[lex111]

Motivation

Fix minor errors related with a11y which reported by Rocket Validator:

• The element “input” must not appear as a descendant of an element with the attribute “role=button” (Toggle)
• Possible misuse of “aria-label” (Showcase)
• Select element must have an accessible name (Showcase)

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Preview.

Related PRs

(If this PR adds or changes functionality, please take some time to update the docs at https://github.com/facebook/docusaurus, and link to your PR here.)

[netlify]

✔️ [V2]

🔨 Explore the source changes: 0c703b8

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/60ba222c192b180008b43677

😎 Browse the preview: https://deploy-preview-4906--docusaurus-2.netlify.app

[netlify]

✔️ [V1]

🔨 Explore the source changes: 0c703b8

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-1/deploys/60ba222c7a3dea0009ead4ca

😎 Browse the preview: https://deploy-preview-4906--docusaurus-1.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	56
🟢 Accessibility	97
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-4906--docusaurus-2.netlify.app/

[github-actions]

Size Change: 0 B

Total Size: 621 kB

ℹ️ View Unchanged

Filename	Size	Change
website/build/assets/css/styles.********.css	88.3 kB	0 B
website/build/assets/js/main.********.js	443 kB	0 B
website/build/blog/2017/12/14/introducing-docusaurus/index.html	62.3 kB	0 B
website/build/docs/introduction/index.html	235 B	0 B
website/build/index.html	27.1 kB	0 B

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[slorber]

👍