docs: fix typos and update links to HTTPS/legacy subdomain

[AliBaig-xD]

This PR fixes minor documentation issues in the /src/content/ folder:

• Corrected possessive typo "it's" → "its" in typescript.md
• Updated outdated HTTP links → HTTPS in conferences.md and data-fetching-with-react-server-components.md
• Updated outdated reactjs.org link → legacy.reactjs.org in StrictMode.md

All changes are low-risk, do not touch any React engine code, and improve clarity and correctness for readers.

No issues referenced.

Guidelines followed:

• ✅ Text style consistent with surrounding content
• ✅ Markdown highlighting (js) preserved where applicable
• ✅ Semantic, clear commit message and PR title
• ✅ Contribution guide and code of conduct followed

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Size changes

📦 Next.js Bundle Analysis for react-dev

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This PR introduced no changes to the JavaScript bundle! 🙌

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