.NET: [Feature]: Include a CHANGELOG.md for dotnet folder like python folder has.

### Description

## Describe the feature you're requesting

Add a `CHANGELOG.md` to the `/dotnet` folder, following the same format and conventions as the existing `/python/CHANGELOG.md`.

## What problem does it solve?

The Python folder has a well-maintained `CHANGELOG.md` (following [Keep a Changelog](https://keepachangelog.com/en/1.0.0/) / SemVer) that makes it easy to see what changed between releases — broken down by package, with PR links, and categorized as Added / Changed / Fixed.

The `dotnet` folder has no equivalent. As a .NET consumer, when a new NuGet package version is released, there is no easy way to know:
- What was added, changed, or fixed
- Which packages were affected
- Whether there are any breaking changes

This makes it difficult to safely upgrade and understand the impact of new releases without manually digging through commits or PRs.

## Expected behavior

A `/dotnet/CHANGELOG.md` file exists and is kept up to date with each release, mirroring the structure of `/python/CHANGELOG.md`:
- Versioned sections (e.g. `## [1.3.0] - 2026-05-07`)
- Entries categorized as `Added`, `Changed`, `Fixed`, `Deprecated`, `Removed`
- Per-package callouts (e.g. `**Microsoft.AgentFramework.Core**:`)
- Links to the relevant PRs

## Alternatives considered

- Reading through GitHub releases/tags — these don't exist in a structured, per-package way for dotnet
- Scanning commits manually — not scalable or consumer-friendly
- The existing `/dotnet/README.md` — this only covers setup, not release history

### Code Sample

```markdown

```

### Language/SDK

.NET