docs: Add Code of Conduct and banner image

This commit introduces a `CODE_OF_CONDUCT.md` to establish community standards and adds a new banner image to the documentation assets.

- **`CODE_OF_CONDUCT.md`**:
    - Adds the Contributor Covenant (v2.1) to foster a welcoming and inclusive community.
    - Defines standards for acceptable behavior, enforcement responsibilities for community leaders, and a clear process for reporting and handling violations.

- **Banner Image**:
    - Adds a new PNG banner image (`banner.png`) to the `docs/img` directory for use in project documentation.

Author: hi-manshu

mkdocs.yml

@@ -104,6 +104,7 @@ nav:
       - Advanced Examples: examples/advanced.md
   - API Reference: api-reference.md
   - Contributing: contributing.md
+  - Code of Conduct: CODE_OF_CONDUCT.md
   - License: license.md
 
 extra: