feat: add Doubao platform support

[rennzhang]

Description

Add first-class Doubao support across the extension and website docs. This registers a new doubao core plugin (extractor, injector, types, tests), wires it into plugin registration and host permissions, and updates EN/ZH supported-platform content, landing page entries, and structured-data text.

Fixes #N/A

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published

Screenshots (if applicable)

N/A