feat(v2): ability to "escape" JSX in MDX files as code blocks

[slorber]

Motivation

Crowdin just added support for the .mdx extension.
It is not real MDX support, but they now interpret .mdx files as md instead of plain text.

The problem is that external tools like Crowdin totally mess-up with the JSX when we download back the translations.

Simple JSX like <UserName name="seb"/> works (ie string props), but as soon as you start to use prop={{x,y,z}} it messes-up.

The idea:

• Ability to wrap problematic jsx fragments into a special code block
• Crowdin sees a code block on mdx source upload => leaves it untouched, does not try to parse it as html
• Add a remark plugin to "unwrap" the code blocks so that JSX fragments remains interpreted inside Docusaurus

Note wrapping in this escape code block is an escape hatch for the mdx + Crowdin (or maybe other saas?) integrations and should be avoided in most cases, as it:

• opt-out of mdx syntax highlighting in IDEs
• opt-out of Prettier formatting the JSX fragments

@yangshun and Titus from MDX seemed to agree it is a viable workaround

Open for other names for the code block, currently choose: mdx-code-block

Can validate that the current doc changes will work for Crowdin to not mess-up with our own site.
Will implement tomorrow the remark plugin

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Size Change: 0 B

Total Size: 544 kB

ℹ️ View Unchanged

Filename	Size	Change
website/build/assets/css/styles.********.css	87.1 kB	0 B
website/build/assets/js/main.********.js	370 kB	0 B
website/build/blog/2017/12/14/introducing-docusaurus/index.html	60.5 kB	0 B
website/build/docs/introduction/index.html	235 B	0 B
website/build/index.html	25.6 kB	0 B

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⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟢 Performance	92
🟢 Accessibility	96
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

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