docs: add Web Collection Agent documentation (zh/en)

[xubinrui]

Summary

• Add Web Collection Agent documentation in both Chinese and English
• Chinese: docs/zh/notes/guide/agent/web_collection.md
• English: docs/en/notes/guide/agent/web_collection.md
• Register the new page in both zh and en sidebar configurations (guide.ts)

Content

The document covers:

1. Overview: Core capabilities of the Web Collection Agent (web search, multi-platform download, dual-channel parallel collection, adaptive data mapping)
2. System Architecture: Detailed node descriptions organized by workflow phases (task analysis, parallel data collection, download, mapping)
3. State Management & Output: WebCollectionState/WebCollectionRequest fields and output file structure
4. User Guide: Gradio UI steps, CLI script usage, Python API, practical case, and notes

Test Plan

• vuepress build passes with 515 pages rendered successfully
• No build errors or warnings