.NET: Removing old code-gen docs from dotnet root

[alliscode]

This pull request removes three design and implementation documents related to the .NET workflow executor source generator feature. The deleted files provided implementation details that are not needed and are more likely to cause confusion where they are.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull request overview

This pull request removes outdated design/implementation markdown documents from the dotnet/ repo root to avoid confusion around the .NET workflow executor source generator feature.

Changes:

• Deleted dotnet/wf-source-gen-changes.md
• Deleted dotnet/wf-source-gen-bp.md
• Deleted dotnet/wf-code-gen-impact.md

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
dotnet/wf-source-gen-changes.md	Removed legacy implementation summary doc for the workflow executor route source generator.
dotnet/wf-source-gen-bp.md	Removed best-practices review doc (Roslyn cookbook comparison) for the source generator.
dotnet/wf-code-gen-impact.md	Removed rationale/impact doc describing the reflection-to-source-gen migration.