Use testing team for docs ownership

[Evangelink]

Summary

• Replace the individual test-framework owners for docs/core/testing/ with @dotnet/dotnet-testing.
• Keep @meaghanlewis as a testing docs owner.

Prerequisite

Before this PR merges, grant @dotnet/dotnet-testing write access to dotnet/docs. The team currently has pull access only, so GitHub reports it as an unknown CODEOWNER.

[Copilot]

Pull request overview

This PR updates documentation ownership routing by consolidating the CODEOWNERS entry for docs/core/testing/ under the @dotnet/dotnet-testing team while retaining @meaghanlewis as an owner, aligning testing docs maintenance with the testing team.

Changes:

• Updated the /docs/core/testing/ CODEOWNERS entry to use @dotnet/dotnet-testing instead of individual owners.
• Kept @meaghanlewis as an explicit CODEOWNER for testing docs.

Show a summary per file

File	Description
.github/CODEOWNERS	Reassigns /docs/core/testing/ ownership to the testing team while retaining an individual owner.

Copilot's findings

• Files reviewed: 1/1 changed files
• Comments generated: 1

[Evangelink]

@BillWagner do you know how I could request our team alias to have write access to the repo? I know how to request access for a single person but not for a team.