docs: comprehensive documentation update for recent features

[teknium1]

Summary

Audited the past 6 days of changes against the Docusaurus documentation site and filled all identified gaps.

New Documentation

• DingTalk messaging guide — complete setup guide for the DingTalk gateway adapter (Stream Mode, env vars, troubleshooting)

Updated Documentation

Providers:

• Added Alibaba Cloud (DashScope), Kilo Code, and Vercel AI Gateway to quickstart provider table
• Added Alibaba Cloud to configuration.md provider table and Chinese providers section
• Added DASHSCOPE_BASE_URL to environment-variables reference

Browser:

• Added Browser Use cloud provider documentation (alternative to Browserbase)
• Added /browser connect CDP mode documentation (attach to local Chrome)
• Updated browser architecture description to reflect multi-provider support
• Fixed contradictory Limitations section that said 'no local browser fallback' while the Overview described one
• Added BROWSER_USE_API_KEY and BROWSER_CDP_URL to env vars reference

Messaging Gateway:

• Added DingTalk, Mattermost, Matrix to architecture diagram
• Added all three platforms to toolset table, security allowlists, and Next Steps links
• Added Mattermost env vars (URL, token, allowed_users, home_channel, reply_mode) to reference
• Added Matrix env vars (homeserver, access_token, user_id, password, allowed_users, home_room, encryption) to reference
• Added DingTalk allowed_users env var to reference
• Added Mattermost, Matrix, DingTalk to sidebar navigation

Configuration:

• Added display.theme_mode (auto/light/dark) documentation
• Added website blocklist configuration section
• Added smart approvals configuration section (ask/smart/off modes)

Security:

• Added website access policy documentation with config example

Slash Commands:

• Updated /tools entry to document enable/disable/list subcommands
• Added /browser connect/disconnect/status command

Build Verification

Docusaurus build passes cleanly with all changes.