[Epic] Follow-ups after the LRO and flatten mitigation are done #3602
Description
After the breaking mitigation is done: #3577, we should update docs accordingly.
- run existing flatten cases to fix any issues
- if the spec is released without flatten, don't introduce breakings
- update the migration guide to remove the flatten part: https://github.com/Azure/azure-sdk-for-js/blob/main/documentation/migration-guide-for-libraries-generated-from-TypeSpec.md
- create its short link and getstarted link
- link the migraiton guide in lro legacy method's description: Support the legacy LRO methods with feature flag #3593 (comment)