docs(v2): Docs refactoring and reorganization

[slorber]

Motivation

Our doc is quite messy, we should re-organize it so that it's easier to navigate in.

Some says that there are "4 kind of docs" (https://documentation.divio.com/):

Not everybody will agree with this vision (after discussions on WriteTheDocs slack), but still probably a good start.

A low-hanging fruit is to split very large doc pages into smaller ones, and move all reference/api doc to the API tab.
It's better to have 1 page per plugin than to have a very large plugins page.

[netlify]

Deploy preview for docusaurus-2 ready!

Built with commit 23ad5f4

https://deploy-preview-3831--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	80
🟢 Accessibility	99
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-3831--docusaurus-2.netlify.app/classic/

[github-actions]

Size Change: +1.48 kB (0%)

Total Size: 157 kB

Filename	Size	Change
website/build/main.********.js	113 kB	+1.5 kB (1%)

ℹ️ View Unchanged

Filename	Size	Change
website/build/blog/2017/12/14/introducing-docusaurus/index.html	20.7 kB	-8 B (0%)
website/build/docs/introduction/index.html	180 B	0 B
website/build/index.html	5.86 kB	-13 B (0%)
website/build/styles.********.css	17.4 kB	0 B

_{compressed-size-action}

[slorber]

Wonder what you think about this refactor @lex111 @Simek @yangshun

Affected pages:

• API tab: https://deploy-preview-3831--docusaurus-2.netlify.app/classic/docs/cli
• Migration guide: https://deploy-preview-3831--docusaurus-2.netlify.app/classic/docs/migration

[lex111]

👏

[Simek]

I really like the API sidebar reorganization and packages emoji marks, good work on that! 🙂

[slorber]

glad you like it :)

let's merge then