update banner for Academy (langchain-ai#578)

## Overview
Added banner for Academy course publicity

## Type of change

**Type:** Added banner for Academy course publicity

## Related issues/PRs
<!-- 
Link to related issues, feature PRs, or discussions (if applicable)

To automatically close an issue when this PR is merged, use closing
keywords:
- "closes langchain-ai#123" or "fixes langchain-ai#123" or "resolves langchain-ai#123"

For regular references without auto-closing, just use:
- "langchain-ai#123" or "See issue langchain-ai#123"

Examples:
- closes langchain-ai#456 (will auto-close issue langchain-ai#456 when PR is merged)
- See langchain-ai#789 for context (will reference but not auto-close issue langchain-ai#789)
-->
- GitHub issue:
- Feature PR:

<!-- For LangChain employees, if applicable: -->
- Linear issue:
- Slack thread:

## Checklist
<!-- Put an 'x' in all boxes that apply -->
- [ ] I have read the [contributing guidelines](README.md)
- [ ] I have tested my changes locally using `docs dev`
- [ ] All code examples have been tested and work correctly
- [ ] I have used **root relative** paths for internal links
- [ ] I have updated navigation in `src/docs.json` if needed
- [ ] I have gotten approval from the relevant reviewers
- [ ] (Internal team members only / optional) I have created a preview
deployment using the [Create Preview Branch
workflow](https://github.com/langchain-ai/docs/actions/workflows/create-preview-branch.yml)

## Additional notes
<!-- Any other information that would be helpful for reviewers -->

Author: lnhsingh

src/docs.json

@@ -104,6 +104,10 @@
   "styles": [
     "/hide-version-picker.css"
   ],
+  "banner": {
+    "content": "Our new LangChain Academy course on Deep Agents is now live! [Enroll for free](https://academy.langchain.com/courses/deep-agents-with-langgraph/?utm_medium=internal&utm_source=docs&utm_campaign=q3-2025_deep-agents-course_co).",
+    "dismissible": true
+  },
   "navbar": {
     "links": [
       {