Update docs.yml to install mkdocs-redirects

docs/source/contributing.md

@@ -64,6 +64,7 @@ other ways)
 
 ```
 pip install mkdocs-material
+pip install mkdocs-redirects
 ```
 
 Then, in the `root` directory of flatbuffers, run 

docs/source/cpp.md

@@ -0,0 +1,2 @@
+# Language Guide: C++
+