DOC: Add code_guidelines to development/index.rst

### Summary

This commit updates `doc/source/development/index.rst` to include a reference to the newly added `code_guidelines.md` document. The goal is to make the unified coding guideline document visible in the Pandas developer documentation sidebar.

### Why This Matters

- Previously, `code_guidelines.md` was added under `doc/source/development/` to address Issue #33851.
- Without referencing it in `index.rst`, the file would not appear in the built documentation.
- This change ensures the document is accessible to all contributors via the official Pandas documentation site.

### What Changed

- Added `code_guidelines` to the existing `.. toctree::` directive.
- Positioned it below `contributing_codebase` as it logically relates to coding standards.

### Follow-Up

This completes the documentation improvement proposed in #33851. All checks have passed, and the document is now correctly linked in the navigation structure.

Author: Abu Jabar Mubarak

doc/source/development/index.rst

@@ -16,6 +16,7 @@ Development
     contributing_environment
     contributing_documentation
     contributing_codebase
+    code_guidelines
     maintaining
     internals
     copy_on_write