Remove deprecated jest-native from ecosystem docs

[Copilot]

jest-native has been deprecated for an extended period but was still surfaced as an active ecosystem package in the docs. This change removes it from current ecosystem navigation and updates related guidance so the docs no longer direct users to deprecated package docs.

• Remove active ecosystem entry

  • Deletes the dedicated jest-native ecosystem page
  • Removes jest-native from the Ecosystem sidebar section

• Remove cross-links from current docs

  • Removes the jest-native companion-library entry from the DOM Testing Library install page

• Align React Native guidance with current matcher support

  • Updates the React Native example to reference built-in matcher support in React Native Testing Library instead of @testing-library/jest-native

- // Using `toHaveTextContent` matcher from `@testing-library/jest-native` package.
+ // Using the built-in `toHaveTextContent` matcher from React Native Testing Library.

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