Extract developer guide snippets into demos

[shai-almog]

Summary

• update the developer guide demos project to Java 17 and Codename One 8.0-SNAPSHOT
• move guide source blocks into demo-backed include files so Java/CSS/native/platform snippets are validated from real files
• compile generated Java snippets and Codename One CSS through the docs/demos build
• add validation for include tags, Android demo sources, and unused guide images

Scope

• Guide images under docs/developer-guide/img are intentionally untouched.
• No screenshot baselines are committed in this PR.
• The workflow does not compare or commit generated screenshots; screenshot refreshes should be handled in later chapter-specific PRs, with generated images discarded or uploaded as artifacts.

Local validation

• python3 scripts/developer-guide/validate-guide-snippets.py
• scripts/developer-guide/compile-android-demo-sources.sh
• mvn -B -ntp -Dgenerate-gui-sources-done=true -Dcodename1.platform=javase -f docs/demos/pom.xml process-classes
• python3 scripts/developer-guide/find_unused_images.py docs/developer-guide --allow-unused img/autocomplete.png --allow-unused img/native-theme-menu.png --allow-unused img/tabs.png
• git diff --check

[github-actions]

Cloudflare Preview

• URL: https://pr-5340-website-preview.codenameone.pages.dev
• Branch: pr-5340-website-preview

[github-actions]

Developer Guide build artifacts are available for download from this workflow run:

• Developer Guide HTML package
• Developer Guide PDF
• AsciiDoc linter report
• Vale report
• Unused image report
• Paragraph capitalization report
• LanguageTool report

Developer Guide quality checks:

• AsciiDoc linter: No issues found (report)
• Vale: No alerts found (report)
• Paragraph capitalization: No paragraph capitalization issues (report)
• LanguageTool: No grammar matches (report)
• Image references: No unused images detected (report)