fix(v2): fix some docs container/sidebar layout issues

[slorber]

Motivation

Fix some docs layout issues.

@CovalentBond reported here that the TOC should take more space for medium size devices (ex: 1300px): #4995 (comment)

Also fixing the collapsed sidebar icon appearing on mobile:

Also fixing the vertical scrollbar appearing for medium width when toc is large:

@lex111 any concern with those changes?

[netlify]

✔️ [V2]

🔨 Explore the source changes: f9fe66b

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/60d1e8822e8cbd0008358997

😎 Browse the preview: https://deploy-preview-5035--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	68
🟢 Accessibility	98
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-5035--docusaurus-2.netlify.app/

[github-actions]

Size Change: -194 B (0%)

Total Size: 569 kB

Filename	Size	Change
website/build/assets/css/styles.********.css	87.6 kB	-194 B (0%)

ℹ️ View Unchanged

Filename	Size
website/build/assets/js/main.********.js	385 kB
website/build/blog/2017/12/14/introducing-docusaurus/index.html	65.8 kB
website/build/docs/introduction/index.html	235 B
website/build/index.html	30.5 kB

_{compressed-size-action}

[lex111]

It's really better this way and it also reduce CSS code.

Also fixing the collapsed sidebar icon appearing on mobile

Strange, can't reproduce it, must be specific case?

[slorber]

Strange, can't reproduce it, must be specific case?

Edge case: you have to collapse the navbar and then reduce your viewport size. Not very likely to happen in practice