Document .NET changelog requirements in SDK Release Prerequisites guide

## Problem

During investigation of a .NET SDK generation scenario, we found a gap between the [Engineering Hub Prerequisites to release client libraries (SDK) documentation](https://eng.ms/docs/products/azure-developer-experience/develop/sdk-release/sdk-release-prerequisites) and the actual .NET release workflow.

The current documentation does not clearly call out that .NET SDK releases may require manual updates to CHANGELOG.md (and related versioning information) before a package can be released. This can create confusion for service teams because other language workflows automate more of these release-preparation steps.

## Additional Finding

As part of this investigation, I verified that the agentic PR review workflow for .NET appears to include changelog validation/review checks and can remind contributors when changelog updates are required. This provides a safeguard during PR review, but the prerequisite documentation still does not set the expectation that manual changelog updates may be required before release.
https://github.com/Azure/azure-sdk-for-net/pull/60064#pullrequestreview-4530872414

Start with the linked SDK Release Prerequisites guide and compare its .NET release guidance with the documented finding about azure-sdk-for-net#60064. Done means the guide clearly states that .NET releases may require manual CHANGELOG.md and related versioning updates before package release, while noting the review workflow safeguard.

Written by the indexing model from the issue text.