Add act documentation

[jackpope]

Content from legacy docs https://legacy.reactjs.org/docs/test-utils.html#act

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Name	Status	Preview	Comments	Updated (UTC)
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[rickhanlonii]

Can you update this to match our other Reference pages?

The top section should be a one-line short description, followed by a psudoish code example, and a TOC:

Then a Reference with a short example, parameters, returns, and caveats

Then a usage section with headings that explain the use case and are visible in the right sidebar:

Then a troubleshooting section:

We can defer the troubleshooting section, but it would include things like not using await or not setting the IS_ACT_ENV global.

[rickhanlonii]

And can we change the base branch to main? Since this was added to 18.3, it should be added to those docs. I'll merge it into the v19 branch

[github-actions]

Size changes

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Page	Size (compressed)	First Load
/[[...markdownPath]]	82.63 KB (🟡 +6 B)	186.78 KB

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