Add chapter about enabling React 19

[TatuLund]

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[github-actions]

AI Language Review

In the updated AsciiDoc file:

1. Under the section "Enable React 19":

   • The phrase "Hilla uses by default React 18, but is tested against both React 18 and 19" can be clarified for better readability. Consider rephrasing it to: "By default, Hilla uses React 18 but is tested against both React 18 and 19."

2. Also under "Enable React 19":

   • Instead of "Thus you can enable React 19 using feature flag," consider: "To enable React 19, use a feature flag."

These changes enhance clarity and ensure a smoother reading experience.