Improve RTD testing docs and add uv guide

[ericholscher]

Summary

This updates user documentation around build integrations on Read the Docs.

• Adds a focused guide for using uv on Read the Docs with reproducible and non-reproducible workflows.
• Links the new uv guide from the build guides index.
• Updates build customization docs to point to the dedicated uv guide and pins uv commands to a specific version.
• Expands the testing guide with Read the Docs-focused integration examples for Vale, Doc Detective, and strict Sphinx warning handling via .readthedocs.yaml.

Validation

• Pre-commit checks passed as part of commit.

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[read-the-docs-community]

Documentation build overview

📚 dev | 🛠️ Build #31754157 | 📁 Comparing 8c31c7c against latest (124231b)

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Show files changed (4 files in total): 📝 4 modified | ➕ 0 added | ➖ 0 deleted

File	Status
contribute.html	📝 modified
install.html	📝 modified
search-integration.html	📝 modified
style-guide.html	📝 modified

[ericholscher]

Superseded by split PRs:\n- #12837 (uv docs)\n- #12838 (testing docs)