misc: more issue template improvements

[slorber]

Motivation

• Replace old canny links: https://docusaurus-2.netlify.com/feedback/
• Github feature requests should be detailed and with a design proposal
• Canny features for basic ideas with no design proposal
• More attempts to protect maintainer's time in general

Have you read the Contributing Guidelines on pull requests?

yes

[netlify]

✔️ [V2]

🔨 Explore the source changes: 795632f

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/60cb1b218f29f00008c5a362

😎 Browse the preview: https://deploy-preview-4994--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	65
🟢 Accessibility	97
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-4994--docusaurus-2.netlify.app/

[github-actions]

Size Change: 0 B

Total Size: 624 kB

ℹ️ View Unchanged

Filename	Size	Change
website/build/assets/css/styles.********.css	87.6 kB	0 B
website/build/assets/js/main.********.js	445 kB	0 B
website/build/blog/2017/12/14/introducing-docusaurus/index.html	63.3 kB	0 B
website/build/docs/introduction/index.html	235 B	0 B
website/build/index.html	28.1 kB	0 B

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