Add a Modernizing legacy code article to the Copilot documentation (#54644)

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
Co-authored-by: hubwriter <hubwriter@github.com>

Author: 3 people

content/copilot/using-github-copilot/guides-on-using-github-copilot/index.md

@@ -12,4 +12,5 @@ children:
   - /refactoring-code-with-github-copilot
   - /using-copilot-to-migrate-a-project
   - /writing-tests-with-github-copilot
+  - /modernizing-legacy-code-with-github-copilot
 ---