Add autoscaling cookbook recipe

[samdark]

Adds a deployment cookbook article for preparing Yii applications for autoscaling: stateless web replicas, shared sessions/cache/uploads, health endpoints, runtime path expectations, migration flow, and worker/scheduler separation. Updates both cookbook navigation locations.

Verification:

• Reproduced the health action in a fresh yiisoft/app template at /tmp/yii-docs-upload-app
• php verify-health-check-action.php
• php -l src/Web/Health/HealthCheckAction.php
• php -l verify-health-check-action.php
• vendor/bin/phpstan analyse src/Web/Health/HealthCheckAction.php verify-health-check-action.php --no-progress
• vendor/bin/psalm --no-progress --output-format=console src/Web/Health/HealthCheckAction.php verify-health-check-action.php
• npx markdown-link-check src/cookbook/deployment/autoscaling.md
• npm run build

[Copilot]

Pull request overview

Adds a new cookbook recipe describing how to prepare a Yii3 application for autoscaling (stateless web instances, shared sessions/cache/uploads, health endpoints, runtime path expectations, single-source migrations, and separation of web/worker/scheduler roles), and wires it into the English cookbook index and VitePress sidebar.

Changes:

• New cookbook article src/cookbook/deployment/autoscaling.md with guidance and example code.
• English cookbook index updated to list the new article.
• English VitePress cookbook sidebar updated with the new entry.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/cookbook/deployment/autoscaling.md	New autoscaling recipe with code samples and checklist.
src/cookbook/index.md	Adds link to the new recipe in the English index.
src/.vitepress/config.js	Adds the new recipe to the English cookbook Deployment sidebar.