feat(docs): add bilingual Parsar documentation site

[AllureCurtain]

What & why

Add an independent bilingual documentation site for Parsar under apps/docs, based on the project's actual architecture and supported workflows. The site provides English and Chinese documentation for getting started, core concepts, Parsar workflows, self-hosting, agent creation, approvals, runs, logs, and troubleshooting.

This gives users a clear product documentation entry point without coupling the docs site to the Go-served Web SPA.

How

• Added a standalone Next.js + Fumadocs MDX app under apps/docs.
• Added English and Chinese documentation pages.
• Added locale-aware routing and language switching.
• Preserved the selected locale when navigating between pages.
• Added grouped sidebar navigation:
  • Getting Started
  • Core Workflows
• Added Parsar favicon and GitHub navigation icon.
• Disabled the development indicator in the docs UI.
• Added docs-specific typecheck and production build checks.
• Updated CI and contributor documentation for the new docs package.
• Updated pnpm-lock.yaml for the new workspace package and its dependencies.

Verification

• make check
• Relevant E2E target: Not applicable
• Manual smoke:
  • Verified / and /zh routes.
  • Verified English/Chinese locale switching.
  • Verified locale-preserving navigation.
  • Verified grouped sidebar navigation.
  • Verified GitHub link and favicon.
  • Verified docs production build successfully generated all pages.

Notes for reviewers

• The docs app is intentionally an independent deployable unit under apps/docs.
• The initial documentation content is based on the current repository implementation.
• A custom docs domain and deployment configuration are intentionally out of scope for this PR.

[liyangbing]

review done