Add terraform-plugin-testing documentation for query mode and query checks

[stephybun]

What

This PR adds documentation for the newly introduced query testing mode as well as the accompanying query checks

What was added:

• New section specific to query-checks containing
• New TestSteps subsection for query mode

What changed:

• TestSteps page was updated to include mentions and references to the new query mode

Why

Terraform introduced a new language construct which users will likely want to know how to test.

Screenshots

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Merge Checklist

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Pull Request

• Description links to related pull requests or issues, if any.

Content

• You added redirects to content/terraform-docs-common/redirects.jsonc for moved, renamed, or deleted pages across all affected versions. Refer to Redirects for examples and guidance.
• Links to related content where appropriate (e.g., CLI, language, API endpoints, permissions, etc.).
• Pages with related content are updated and link to this content when appropriate.
• Sidebar navigation files have been updated for added, deleted, reordered, or renamed pages.
• New pages have metadata (page name and description) at the top.
• New images are 2048 px wide. They have HashiCorp standard annotation color (#F92672) and format (rectangle with rounded corners), blurred sensitive details (e.g. credentials, usernames, user icons), and descriptive alt text in the markdown for accessibility.
• New code blocks have the correct syntax and line breaks to eliminate horizontal scroll bars.
• UI elements (button names, page names, etc.) are bolded.
• The Vercel website preview successfully deployed.

Reviews

• I or someone else reviewed the content for technical accuracy.
• I or someone else reviewed the content for typos, punctuation, spelling, and grammar.

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