.NET: Add additional build, test and project structure skills

[westey-m]

Motivation and Context

Copilot tends to build the entire project every time, which can be very time consuming.

Description

• Adding some skills to describe the project structure and provide tips on doing smaller more targeted builds and tests.

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 PR adds documentation skills to help GitHub Copilot perform more targeted builds and tests of the .NET codebase. The changes address the issue where Copilot tends to rebuild the entire solution every time, which is time-consuming for this large repository.

Changes:

• Fixed YAML front matter formatting in the verify-dotnet-samples skill
• Added new project-structure skill documenting the .NET solution folder organization
• Added new build-and-test skill with examples for targeted builds and test execution

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
dotnet/.github/skills/verify-dotnet-samples/SKILL.md	Removed line break in YAML description field to fix formatting
dotnet/.github/skills/project-structure/SKILL.md	New skill documenting src/, tests/, samples/, Shared/, and LegacySupport folder structure
dotnet/.github/skills/build-and-test/SKILL.md	New skill with guidance on targeted builds, test execution, and multi-framework targeting tips