docs: Add Leap0 sandbox integration docs

[Julia Passynkova (ipassynk)]

Overview

Adds documentation for the Leap0 sandbox integration with LangChain Deep Agents:

• Sandbox integration page for langchain-leap0 and Leap0Sandbox (install, backend setup, Deep Agents example, cleanup, LEAP0_API_KEY).
• Provider hub page under Python integrations linking to the sandbox doc.
• leap0-icon.svg for cards and UI consistency.
• Deep Agents sandboxes page: Leap0 cards with links to /oss/integrations/sandboxes/leap0 and the Leap0 icon.

Type of change

Type: New documentation page

Checklist

• [x ] I have read the contributing guidelines (README.md), including the language policy (https://docs.langchain.com/oss/python/contributing/overview#language-policy)
• [ x] I have tested my changes locally using docs dev
• [ x] All code examples have been tested and work correctly
• [x ] I have used root relative paths for internal links
• I have updated navigation in src/docs.json if needed

[Julia Passynkova (ipassynk)]

Mason Daugherty (@mdrxy) Lauren Hirata Singh (@lnhsingh) Can you please take a look at this PR? This is integration of LangChain and leap0.dev (Enterprise-gradecloud sandboxes for AI agents). Here is langchain-leap0 github - https://github.com/leap0-dev/langchain-leap0 Thank you