Add documentation contribution guidance to opening page

Added "Improving the Documentation" section explaining:
- How to use the Report Issue footer icon (pre-filled with page URL/title)
- How to use Edit on GitHub button for direct edits

Underworld development team with AI support from Claude Code

Author: lmoresi

docs/index.md

@@ -151,11 +151,19 @@ Underworld3 is a Python library for computational geodynamics, built on:
 
 ### Getting Help
 
-- [Report issues on GitHub](https://github.com/underworldcode/underworld3/issues)
 - [Browse API documentation](api/index.md)
+- [Report issues on GitHub](https://github.com/underworldcode/underworld3/issues)
 - Join community discussions
 - [Underworld Blog](https://www.underworldcode.org)
 
+### Improving the Documentation
+
+Found a typo, broken link, or have a suggestion? We welcome contributions to improve the documentation.
+
+**Report an issue**: Click the "Report Issue" icon in the page footer. This opens a GitHub issue pre-filled with the current page URL and title.
+
+**Edit directly**: Click the "Edit on GitHub" button (top right of any page) to propose changes directly via a pull request.
+
 ### Try It Now
 
 Launch interactive tutorials directly in your browser: