Document variable path segment routes

[samdark]

Summary

• add routing guide examples for bounded variable-depth routes with nested optional parts
• document how arbitrary-depth regex route arguments behave
• warn that generated catch-all route arguments encode slashes as %2F, so optional parts are better for generated hierarchical URLs

Verification

• Used a fresh yiisoft/app template created with composer create-project yiisoft/app /tmp/yii-docs-upload-app --no-interaction
• Added verify-variable-url-parameters.php to exercise Yii Router matching and URL generation
• Ran php -l verify-variable-url-parameters.php
• Ran php verify-variable-url-parameters.php; verified:
  • /docs
  • /docs/guide
  • /docs/guide/routing
  • catch-all generation gives /files/2026%2Freport.pdf
  • catch-all matching captures path => 2026/report.pdf
• Ran vendor/bin/phpstan analyse verify-variable-url-parameters.php --level=max --no-progress
• Ran vendor/bin/psalm --no-cache --output-format=console verify-variable-url-parameters.php
• Ran npx markdown-link-check src/guide/runtime/routing.md
• Ran npm run build

[Copilot]

Pull request overview

This PR expands the routing guide with examples for variable-depth URL paths, covering both bounded optional route segments and catch-all regex parameters.

Changes:

• Adds a new routing subsection for variable numbers of path segments.
• Documents nested optional parts for predictable matching and URL generation.
• Explains catch-all regex route behavior and %2F encoding during URL generation.