docs: add RAYON_NUM_THREADS environment variable documentation

[Michael-M-Judd]

Summary

This PR adds documentation for the RAYON_NUM_THREADS environment variable to help users running multiple Biome instances in parallel (e.g., in monorepos or agentic workflows) to limit CPU usage and prevent resource contention.

Changes

• Added caution callout in big-projects.mdx about agentic workflows and parallel processes
• Updated environment_variables.md with RAYON_NUM_THREADS documentation
• Updated environment-variables.md with RAYON_NUM_THREADS documentation
• Updated codegen/src/env_variables.rs to include the new variable in the code generation template

Motivation

When running multiple Biome processes in parallel (e.g., with multiple agents in CI/CD), each instance can use 60-80% CPU, causing resource contention. The --threads flag only works for the ci command, but RAYON_NUM_THREADS works across ALL Biome commands, making it more convenient for limiting resource usage globally.

Testing

• Verified the documentation renders correctly
• Confirmed the code generation template includes the new variable

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Tracked Files

Locale	File	Note
en	src/content/docs/guides/big-projects.mdx	Source changed, localizations will be marked as outdated.
en	src/content/docs/reference/environment-variables.md	Source changed, localizations will be marked as outdated.
en	src/content/docs/reference/environment_variables.md	Source changed, localizations will be marked as outdated.

Warnings reference

Icon	Description
🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.

[coderabbitai]

Walkthrough

This pull request adds documentation for the RAYON_NUM_THREADS environment variable across the documentation and code generation files. The changes include a new reference section describing the variable's purpose (controlling Biome's internal thread pool size via Rayon), its scope (applies to all Biome commands), and usage examples for both per-process and global configuration. Additionally, a caution block is added to the big-projects guide warning about potential high CPU usage when running multiple Biome processes in parallel. No functional code changes are introduced.

Possibly related PRs

• feat: add reference for RUST_BACKTRACE environment variable #3770: Modifies the same documentation FOOTER in codegen/src/env_variables.rs to add a new environment-variable reference (RUST_BACKTRACE).

Suggested labels

A-Documentation

Suggested reviewers

• ematipico

🚥 Pre-merge checks | ✅ 2

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title directly and specifically describes the main change: adding documentation for the RAYON_NUM_THREADS environment variable.
Description check	✅ Passed	The description clearly relates to the changeset, providing context about the RAYON_NUM_THREADS environment variable documentation, motivation, and testing performed.

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✨ Finishing touches

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🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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[coderabbitai]

Actionable comments posted: 0

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⚠️ Outside diff range comments (1)

src/content/docs/reference/environment-variables.md (1)

1-6: ⚠️ Potential issue | 🟠 Major

Fix naming inconsistency between codegen output and documentation routing.

The codegen script writes to environment_variables.md (underscore), but astro.config.ts references environment-variables (hyphen). Both files currently exist with identical content and falsely claim auto-generation. Either update the codegen to write to the hyphen version, update the config to reference the underscore version, or remove the duplicate—but maintain only one source of truth to prevent drift.

[dyc3]

I think I would rather introduce this as an actual feature rather than document this env var, in case we want to move away from rayon in the future. Maybe that doesn't matter that much tho because we could always have it as an alias.