feat(docs): add webchat migration

[janelletam]

Closes DOCS-107

Summary

• Migrate the full Webchat section to docs-v2, including get-started guides, platform integrations (React Native, Webflow, Wix, WordPress), interact API docs, and the React library component reference
• Add all image assets in light/dark variants
• Add new webchat nav section + update routes

Test plan

• Verify webchat get-started pages render: introduction, quick-start, embedding-webchat, embed-in-element, configure-your-webchat
• Check integration guides: React Native, Webflow, Wix, WordPress
• Verify interact docs: error-handling, listen-to-events, reference, send-custom-events, notifications, etc.
• Check React library docs: get-started, hooks, and all component pages (Composer, Container, FAB, Header, MessageList, StylesheetProvider, Webchat)
• Confirm light/dark image variants switch correctly with the theme
• Confirm nav links in sidebar resolve correctly

[greptile-apps]

Too many files changed for review. (113 files found, 100 file limit)

[linear-code]

DOCS-107