Extract docs-toolkit as reusable package

[yomybaby]

Extract the Backend.AI WebUI docs markdown → PDF/HTML engine as a standalone, reusable npm package.

Why This Work is Needed

The current backend.ai-webui-docs package mixes the documentation engine (markdown processing, PDF rendering, preview servers) with Backend.AI-specific content and configuration. This makes it difficult to reuse the same documentation workflow for other projects.

Expected Outcomes

• Reusable backend.ai-docs-toolkit package with CLI, config system, and agent templates
• backend.ai-webui-docs migrated to consume the toolkit as a workspace dependency
• Config-driven architecture allowing customization via docs-toolkit.config.yaml
• Handlebars-based agent template system for generating Claude Code agent files

JIRA Issue: FR-2084