message: [docs] updated with typescript definitions + code sample
diff: The `featureFlags` object passed to the `register` function makes it possible for plugins to register Feature Flags in Backstage for users to opt into. You can use this to split out logic in your code for manual A/B testing, etc.
  ```typescript
  -// This needs to be updated.
  +export type FeatureFlagsHooks = {
  + registerFeatureFlag(name: FeatureFlagName): void;
  +};
  ```
  Then, later, if you want to check if the user has enabled them:
  -```typescript
  -// This needs to be updated.
  +```tsx
  +import React, { FC } from 'react';
  +import { Button } from '@material-ui/core';
  +import { featureFlagsApiRef, useApi } from '@backstage/core';
  +
  +const ExampleButton: FC<{}> = () => {
  + const featureFlagsApi = useApi(featureFlagsApiRef);
  +
  + const handleClick = () => {
  + featureFlagsApi.enable('enable-example-feature');
  + };
  +
  + return (
  + <Button variant="contained" color="primary" onClick={handleClick}>
  + Enable the 'enable-example-feature' feature flag
  + </Button>
  + );
  +};
  ```
  [Back to References](README.md)
  