Polish the public beta community launch

[Jayktopus]

What changed

• adds a compact, professional product demonstration to the English and German landing pages
• restructures both READMEs around installation, workflow, capabilities, limitations and contribution paths
• adds a public roadmap, central known-limitations document and tailored community post drafts
• prepares source-only release notes for 0.3.0-beta.8
• updates package metadata to 0.3.0-beta.8

Why

The public repository needed a clear first impression, honest beta expectations and concrete entry points for users and contributors.

Validation

• 198 unit tests pass
• internal Markdown links checked
• GIF visually inspected and compressed to 5.3 MB
• git diff --check passes

GitHub labels, a Public Beta milestone and eight focused contributor issues were created separately. Obsolete closed-beta releases and tags were removed.