docs: fix typos and broken/outdated links

[AliBaig-xD]

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

• Corrected possessive typo "it's" → "its" in sunsetting-create-react-app.md and react-labs-view-transitions-activity-and-more.md
• Fixed typo "inital mount" → "initial mount" in StrictMode.md
• Updated broken/outdated links (https://18.react.dev//reference/ → https://18.react.dev/reference/) in legacy.md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

[github-actions[bot]]

Size changes

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