Add link to GitHub Copilot app landing page (#61597)

Author: saritai

content/copilot/how-tos/github-copilot-app/index.md

@@ -7,6 +7,9 @@ versions:
 contentType: how-tos
 layout: bespoke-landing
 heroImage: /assets/images/banner-images/hero-4
+sidebarLink:
+  text: All articles
+  href: /copilot/how-tos/github-copilot-app
 introLinks:
   overview: /copilot/concepts/agents/github-copilot-app
   quickstart: /copilot/how-tos/github-copilot-app/getting-started