chore:(docs) Update API Extractor file (#31651)

docs/react-v9/contributing/api-extractor.md

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+### What is API Extractor?
+
+[API Extractor](https://api-extractor.com/) generates an api.md file containing the public API for a package. `@fluentui/react` and related packages as well as `@fluentui/web-components` use API Extractor to ensure that API changes must be reviewed before being merged into master. (`@fluentui/react-northstar` and related packages do not use API Extractor as of writing.)
+
+### Build Tasks
+
+As part of a package's build process, the api-extractor task checks the package's public API with the package's auto-generated .api.md file. Be sure to commit the updates with your other changes
+
+In PR builds, the task will fail if it detects missing API file updates.
+
+To generate the API markdown file commit your changes, then run:
+
+`yarn nx run <package-name>:build`
+
+Commit and push the updated file.