feat: improve landing page onboarding and documentation experience

[PerfectPan]

Problem

The current landing page does not provide enough guidance for new users.

• GitHub stars are not visible
• There is no dedicated documentation site
• Users cannot easily understand how to use OCVM
• The landing page and docs experience are visually disconnected

Proposed Solution

• Build a dedicated documentation site using Fumadocs:
  https://www.fumadocs.dev/
• Add GitHub star visibility to the landing page
• Unify the visual style, navigation, and branding between the landing page and the documentation site

Alternatives Considered

Impact

• CLI behavior:
• Configuration:
• Compatibility:
• Documentation: