chore: fix codesandbox default privacy

[slorber]

Motivation

CodeSandbox repros we receive lately are private by default, which is annoying:

Apparently, new query string options have been implemented to prevent this, so let's figure what is the best for our use case.

https://twitter.com/CompuIves/status/1773028692752539825

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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🔴 42	🟢 98	🟢 96	🟢 100	🟠 88	Report
/docs/installation	🟠 56	🟢 96	🟢 100	🟢 100	🟠 88	Report
/docs/category/getting-started	🟠 75	🟢 100	🟢 100	🟢 90	🟠 88	Report
/blog	🟠 68	🟢 100	🟢 100	🟢 90	🟠 88	Report
/blog/preparing-your-site-for-docusaurus-v3	🟠 61	🟢 96	🟢 100	🟢 100	🟠 88	Report
/blog/tags/release	🟠 68	🟢 100	🟢 100	🟠 80	🟠 88	Report
/blog/tags	🟠 76	🟢 100	🟢 100	🟢 90	🟠 88	Report

[slorber]

Note to self: from this twitter conversation I understand we can use the new ?fork=true&privacy=public query string.

Using fork=true is not an ideal general UX to me because users get presented with a modal immediately even before seeing Docusaurus running in a sandbox, so let's only use privacy=public alone