docs(v2): Improve intro doc

[slorber]

Motivation

The intro doc is not so good:

• a lot of focus on v1. New users don't care about how v1 worked.
• more focus on playing with Docusaurus asap instead of reading about its design decisions etc

[netlify]

[V2]

Built with commit 463f37d

https://deploy-preview-4770--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	71
🟢 Accessibility	96
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-4770--docusaurus-2.netlify.app/

[netlify]

[V1]

Built with commit 463f37d

https://deploy-preview-4770--docusaurus-1.netlify.app

[github-actions]

Size Change: -352 B (0%)

Total Size: 691 kB

Filename	Size	Change
website/build/assets/js/main.********.js	513 kB	-352 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
website/build/assets/css/styles.********.css	88.5 kB	0 B
website/build/blog/2017/12/14/introducing-docusaurus/index.html	62.1 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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