C++ language-home: use new layout, link to examples and docs (#507)

Contributes to #481, #345, #506

content/docs/languages/cpp/_index.md

@@ -1,10 +1,14 @@
 ---
 title: C++
+layout: prog_lang_home
+src_repo: https://github.com/grpc/grpc
+content:
+  - learn_more:
+    - "[Async-API tutorial](async/)"
+    - "[Additional docs]($src_repo_url/tree/master/doc)"
+    - "[Examples]($src_repo_url/tree/master/examples/cpp)"
+  - reference:
+    - "[API](api/)"
+  - other:
+    - "[grpc repo]($src_repo_url)"
 ---
-
-These language-specific pages are available:
-
-- [Quick start](quickstart/)
-- [Basics tutorial](basics/)
-- [Asynchronous-API tutorial](async/)
-- [API reference](api)