misc: change showcase submission process, use GitHub discussion

[slorber]

Motivation

We are changing the way showcase sites are submitted: instead of users submitting a PR themselves, we just ask them to post a GitHub comment in #7826

This have several benefits:

• it makes it easier for site owners to get their site added,
• it does not require signing the Facebook CLA (which may sometimes be more complicated for companies)
• it avoids the creation of too many PRs not directly related to Docusaurus changes
• it saves a lot of time for the Docusaurus maintainers

[netlify]

✅ [V2]

Name	Link
🔨 Latest commit	e5ecfba
🔍 Latest deploy log	https://app.netlify.com/sites/docusaurus-2/deploys/62dec2ab1ba3bc000735c0f7
😎 Deploy Preview	https://deploy-preview-7828--docusaurus-2.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 82	🟢 100	🟢 100	🟢 100	🟢 90	Report
/docs/installation	🟠 74	🟢 100	🟢 100	🟢 100	🟢 90	Report

[github-actions]

Size Change: 0 B

Total Size: 805 kB

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	52.8 kB
website/build/assets/css/styles.********.css	106 kB
website/build/assets/js/main.********.js	608 kB
website/build/index.html	38.9 kB

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