Fixes #9507 fix(nimbus): Fix experimenter docs links (#9522)

Because

- We had a few broken links to experimenter.info

This commit

- Fixes links for:
- [Rollout bucketing
warning](https://experimenter.info/rollouts/rollouts-bucketing-warning)
- [Custom
audiences](https://experimenter.info/workflow/implementing/custom-audiences)

experimenter/experimenter/nimbus-ui/src/lib/constants.ts

@@ -66,9 +66,9 @@ export const EXTERNAL_URLS = {
   LAUNCH_DOCUMENTATION:
     "https://experimenter.info/access#onboarding-for-new-reviewers-l3",
   BUCKET_WARNING_EXPLANATION:
-    "https://experimenter.info/rollouts-and-experiments#question-2",
+    "https://experimenter.info/rollouts/rollouts-bucketing-warning",
   CUSTOM_AUDIENCES_EXPLANATION:
-    "https://experimenter.info/workflow/custom-audiences",
+    "https://experimenter.info/workflow/implementing/custom-audiences",
   WHAT_TRAIN_IS_IT: "https://whattrainisitnow.com",
 };