docs: fix grammar and spelling errors in documentation.

[zarahzachz]

What I did

1. Fixed grammar and typos in docs.
2. Addressed some Markdown issues (test-runner/typescript.md) where it looked like content was meant to be its own paragraph.

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Latest commit: b457b5b

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[bashmish]

thanks!

[aghArdeshir]

All these typos still exist on prod (https://modern-web.dev/). It could be cool if we publish it. ❤️