Fix #360: Add package design guide

[samdark]

Closes #360.

Summary

• add a guide for designing Yii-aware Composer packages that provide application configuration
• cover config groups, routes, DI providers, params, assets, events, migrations, translations, themes, and console commands
• add the page to the guide TOC and VitePress sidebar

Checks

• npm run build
• npm run check-links -- src/guide/structure/designing-packages.md src/guide/structure/package.md src/guide/index.md

[Copilot]

Pull request overview

Adds a new guide page describing how to design Composer packages that integrate with a Yii3 application via configuration. It walks through package layout, the extra.config-plugin metadata, parameters, DI definitions and providers, routes, events, assets, migrations, translations, themes, console commands, and application-override guidance. The page is wired into the guide index, the VitePress sidebar, and cross-linked from package.md.

Changes:

• Add src/guide/structure/designing-packages.md covering all Yii3 package integration points listed in #360.
• Link the new page from src/guide/structure/package.md and the guide index.
• Register the page in the VitePress sidebar under Structure.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/guide/structure/designing-packages.md	New guide page documenting Yii3 package design across config groups, DI, routes, events, assets, migrations, translations, themes, and commands.
src/guide/structure/package.md	Cross-link from the existing Packages page to the new designing-packages page.
src/guide/index.md	Add the new page to the Structure section of the guide TOC.
src/.vitepress/config.js	Add the new page to the VitePress sidebar under Structure.

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.