Add an entry in navigation bar for python bindings docs (#238)

This is a follow-up PR of #235.

This PR adds a button in the navigation bar for the python bindings API
docs (https://mlir.llvm.org/python-bindings/), so that users can find
the generated Sphinx docs.

cc @jpienaar

Author: PragmaTwice

website/config.toml

@@ -91,6 +91,12 @@ disablePathToLower = true
     weight = 1
     url = "/doxygen/"
 
+[[menu.main]]
+    name = "Python Bindings API docs"
+    parent = "Source"
+    weight = 3
+    url = "/python-bindings/"
+
 [[menu.main]]
     name = "Bugs"
     weight = 20