Chore: Update the logo/readme, make testapp not produce nuget package

[glennawatson]

This pull request primarily updates the packaging configuration for the project to improve the NuGet package metadata and contents. The changes ensure that important files like the README.md and logo.png are correctly included in the package, and that the test project is not packable.

Packaging improvements:

• Added the README.md file to the NuGet package metadata using the PackageReadmeFile property in Directory.Build.props.
• Included both README.md and logo.png in the package contents by updating the ItemGroup in Directory.Build.props.

Test project configuration:

• Marked the test application project (Splat.DI.SourceGenerator.TestApp.csproj) as not packable by setting <IsPackable>false>.

[Copilot]

Pull request overview

This pull request improves the NuGet package configuration by organizing assets and ensuring the test application is not packaged.

Changes:

• Moved logo.png from repository root to images/ directory for better organization
• Added README.md to the NuGet package metadata and contents
• Marked the test application project as non-packable to prevent accidental publication

Reviewed changes

Copilot reviewed 2 out of 4 changed files in this pull request and generated no comments.

File	Description
src/tests/Splat.DI.SourceGenerator.TestApp/Splat.DI.SourceGenerator.TestApp.csproj	Added IsPackable property set to false to prevent test app from being packaged
src/Directory.Build.props	Added PackageReadmeFile property and updated file includes to reference logo.png in images/ directory and include README.md in package
images/logo.png	Logo file added to images/ directory (moved from root)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.41%. Comparing base (7967f28) to head (67e31dd).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #290   +/-   ##
=======================================
  Coverage   94.41%   94.41%           
=======================================
  Files          17       17           
  Lines         896      896           
  Branches      145      145           
=======================================
  Hits          846      846           
  Misses         19       19           
  Partials       31       31           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.