feat: add langchain svg

[imxv]

📝 About your SVG:

• Title: LangChain
• Category: AI, Library
• Website URL: https://www.langchain.com/
• Description: LangChain is an open-source framework (MIT license) for building applications with Large Language Models. Closes [🔔 Request SVG]: Add a Langchain svg #880.

📷 Screenshots

Logo preview (rendered from the added file):

🔗 Source

SVG sourced directly from LangChain's official website (Webflow CDN) — the icon currently used in the header of https://www.langchain.com/. File is unmodified.

✅ Checklist

• I have permission to use this logo (LangChain is MIT-licensed OSS; the file is the exact asset LangChain self-publishes on their homepage).
• The `.svg` file is optimized for web use (550 bytes, single `` root, no metadata/comments).
• The `.svg` size is less than 20kb.