docs: add docs about env to control working threads limit

[hardfist]

Summary

add docs about how to use env to control working threads limit
I tried to use RSPACK_WORKING_THREADS to control both rayon and tokio threads number but rayon doesn't encourge doing so rayon-rs/rayon#878 (comment)

Related links

• [Feature]: Need ability to limit CPU and RAM usage in CI environments #12382

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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[github-actions]

Rsdoctor Bundle Diff Analysis

Found 5 project(s) in monorepo.

📁 react-10k

Path: ../build-tools-performance/cases/react-10k/dist/rsdoctor-data.json

⚠️ No baseline data found - Unable to perform comparison analysis

Metric	Current	Baseline	Change
📊 Total Size	5.7 MB	-	-
📄 JavaScript	5.7 MB	-	-
🎨 CSS	21.0 B	-	-
🌐 HTML	0 B	-	-
📁 Other Assets	0 B	-	-

📁 react-1k

Path: ../build-tools-performance/cases/react-1k/dist/rsdoctor-data.json

⚠️ No baseline data found - Unable to perform comparison analysis

Metric	Current	Baseline	Change
📊 Total Size	823.6 KB	-	-
📄 JavaScript	823.6 KB	-	-
🎨 CSS	0 B	-	-
🌐 HTML	0 B	-	-
📁 Other Assets	0 B	-	-

📁 react-5k

Path: ../build-tools-performance/cases/react-5k/dist/rsdoctor-data.json

⚠️ No baseline data found - Unable to perform comparison analysis

Metric	Current	Baseline	Change
📊 Total Size	2.7 MB	-	-
📄 JavaScript	2.7 MB	-	-
🎨 CSS	21.0 B	-	-
🌐 HTML	0 B	-	-
📁 Other Assets	0 B	-	-

📁 rome

Path: ../build-tools-performance/cases/rome/dist/rsdoctor-data.json

⚠️ No baseline data found - Unable to perform comparison analysis

Metric	Current	Baseline	Change
📊 Total Size	984.3 KB	-	-
📄 JavaScript	984.3 KB	-	-
🎨 CSS	0 B	-	-
🌐 HTML	0 B	-	-
📁 Other Assets	0 B	-	-

📁 ui-components

Path: ../build-tools-performance/cases/ui-components/dist/rsdoctor-data.json

⚠️ No baseline data found - Unable to perform comparison analysis

Metric	Current	Baseline	Change
📊 Total Size	2.1 MB	-	-
📄 JavaScript	2.0 MB	-	-
🎨 CSS	83.0 KB	-	-
🌐 HTML	0 B	-	-
📁 Other Assets	0 B	-	-

Generated by Rsdoctor GitHub Action

[github-actions]

📦 Binary Size-limit

Comparing df543f0 to chore(ci): run Mac CI test on github hosted runner (#12397) by pshu

❌ Size increased by 2.25KB from 47.71MB to 47.72MB (⬆️0.00%)

[codspeed-hq]

CodSpeed Performance Report

Merging #12383 will not alter performance

_{Comparing yj/use-worker-threads (df543f0) with main (9e0280f)}

Summary

✅ 17 untouched

[Copilot]

Pull request overview

This PR adds support for controlling worker thread counts in Rspack through a new RSPACK_WORKER_THREADS environment variable. When set, this variable configures both the tokio runtime worker threads and the rayon global thread pool to use the specified number of threads, providing users with more fine-grained control over Rspack's parallelism.

Key Changes

• Introduces the RSPACK_WORKER_THREADS environment variable to configure both tokio and rayon thread pools
• Refactors tokio runtime builder initialization to support conditional worker thread configuration
• Adds rayon global thread pool initialization when RSPACK_WORKER_THREADS is set

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