6.x: Create mkdocs pages

[matejsemancik]

Summary

• Add MkDocs Material documentation site with full architecture docs for both Android and KMP paths
• Platform-first structure: single Android page, multi-page KMP section (Components, State, Events, Navigation, Factory Generator)
• Include Getting Started guides, Use Cases, Testing, Lint rules, and Migration sections
• Add GitHub Pages deployment workflow
• Replace lengthy README with a short pointer to the docs site

Changes

• mkdocs.yml — Site config with Material theme, macros plugin, and full nav structure
• docs/architecture/ — Android (single page) and KMP (sub-pages) architecture docs
• docs/getting-started/ — Installation, project setup, and quick start for both paths
• docs/use-cases/ — UseCase, FlowUseCase, Result, and error handling docs
• docs/testing/ — ViewModel testing and UseCase mocking
• .github/workflows/publish_pages.yml — GitHub Pages deployment CI

Test plan

• Run mkdocs serve locally and verify all pages render
• Verify no broken links in navigation
• Check GitHub Pages deployment workflow triggers correctly