Add Testing section to README

[SharpRake]

Type of change

Documentation Update

• Add a Testing section to the README explaining how draft content is verified before publication
• Document accuracy review, local preview, pre-commit hooks, and deploy previews

What should this PR do?

resolves https://github.com/chainguard-dev/internal/issues/5865

Why are we making this change?

The README documented local development and contributing, but not how content is tested before it goes live. This section gives contributors and reviewers a shared understanding of the checks every draft passes through, including accuracy review and testing of any commands or code snippets.

What are the acceptance criteria?

• The README includes a Testing section under Development
• The section explains that content is reviewed for accuracy and that commands and code snippets are run and tested by both author and reviewer
• The section describes local preview, pre-commit hooks, and Netlify deploy previews
• The internal link to the Pre-commit Hooks section resolves correctly

How should this PR be tested?

1. Check the preview link and ensure the changes look right
2. Navigate to the README on the branch and confirm the new Testing section renders between Development and Contributing
3. Confirm the "Pre-commit Hooks" link in the Testing section jumps to the correct heading

🤖 Generated with Claude Code

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