fix(v2): tidy up Markdown page layout

[lex111]

Motivation

Quite strange looking now markdown page, I suppose it should be aligned to center like on all the other pages (see screenshots below).

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Before (https://docusaurus.io/examples/markdownPageExample)	After (https://deploy-preview-4917--docusaurus-2.netlify.app/examples/markdownPageExample)

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: 611e063

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/60bcc4306baa95000727cbcb

😎 Browse the preview: https://deploy-preview-4917--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	61
🟢 Accessibility	97
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-4917--docusaurus-2.netlify.app/

[netlify]

✔️ [V1]

🔨 Explore the source changes: 611e063

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-1/deploys/60bcc43063e29f0007525a7f

😎 Browse the preview: https://deploy-preview-4917--docusaurus-1.netlify.app

[github-actions]

Size Change: +44 B (0%)

Total Size: 621 kB

Filename	Size	Change
website/build/assets/css/styles.********.css	88.3 kB	+44 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
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

_{compressed-size-action}

[slorber]

I'm not a designer but find it better if the actual content remains centered.

I don't like too much the idea of having the TOC move the content to the left.

Also, wouldn't this lead to different md page layouts for pages with/without TOC?

Is the content of a page without TOC moved to the left (would be weird)? or in this case, the content is in the center?

I'd rather keep them all looking the same so that it's not confusing when you navigate from a page with TOC to a page without TOC (ie the content remains at the same place)

[lex111]

I just implement the same behavior as on the doc page, hide_table_of_contents=true is useful if the user wants to expand the width of content part:

Default ( hide_table_of_contents=false)	With hide_table_of_contents=true

So I thought it would be helpful on regular pages as well.

[slorber]

Hmmm isn't it the same screenshot? Can't tell the differences.

@yangshun any opinion regarding how the content should be centered?

[lex111]

I wanted to show that if TOC is hidden, then the content column is stretched to container full width.