docs: update missing link (#26041)

Update the links about feature flags in the contribution guide under "Feature Flags" section because these is  missing link.

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: hassaku63

CONTRIBUTING.md

@@ -1095,7 +1095,7 @@ automatically, and we have to use a feature flag:
 Adding a new flag looks as follows:
 
 1. Define a new const under
-   [cx-api/lib/features.ts](https://github.com/aws/aws-cdk/blob/main/packages/%40aws-cdk/cx-api/lib/features.ts)
+   [cx-api/lib/features.ts](https://github.com/aws/aws-cdk/blob/main/packages/aws-cdk-lib/cx-api/lib/features.ts)
    with the name of the context key that enables this new feature (for
    example, `ENABLE_STACK_NAME_DUPLICATES`). The context key should be in the
    form `module.Type:feature` (e.g. `@aws-cdk/core:enableStackNameDuplicates`).
@@ -1106,12 +1106,12 @@ Adding a new flag looks as follows:
 2. Use `FeatureFlags.of(construct).isEnabled(cxapi.ENABLE_XXX)` to check if this feature is enabled
    in your code. If it is not defined, revert to the legacy behavior.
 3. Add your feature flag to the `FLAGS` map in
-   [cx-api/lib/features.ts](https://github.com/aws/aws-cdk/blob/main/packages/%40aws-cdk/cx-api/lib/features.ts). In
+   [cx-api/lib/features.ts](https://github.com/aws/aws-cdk/blob/main/packages/aws-cdk-lib/cx-api/lib/features.ts). In
    your description, be sure to cover the following:
    - Consciously pick the type of feature flag. Can the flag be removed in a future major version, or not?
    - Motivate why the feature flag exists. What is the change to existing infrastructure and why is it not safe?
    - In case of a "default change flag", describe what the user needs to do to restore the old behavior.
-4. Add an entry for your feature flag in the [README](https://github.com/aws/aws-cdk/blob/main/packages/%40aws-cdk/cx-api/README.md) file.
+4. Add an entry for your feature flag in the [README](https://github.com/aws/aws-cdk/blob/main/packages/aws-cdk-lib/cx-api/README.md) file.
 5. In your tests, ensure that you test your feature with and without the feature flag enabled. You can do this by passing the feature flag to the `context` property when instantiating an `App`.
    ```ts
    const myFeatureFlag = { [cxapi.MY_FEATURE_FLAG]: true };
@@ -1124,7 +1124,7 @@ Adding a new flag looks as follows:
 
     `fix(core): impossible to use the same physical stack name for two stacks (under feature flag)`
 
-[jest helper methods]: https://github.com/aws/aws-cdk/blob/main/tools/@aws-cdk/cdk-build-tools/lib/feature-flag.ts
+[jest helper methods]: https://github.com/aws/aws-cdk/blob/main/packages/aws-cdk-lib/core/lib/feature-flags.ts
 
 ## Versioning and Release