[DOCS]Update debugger in docs.tvm.ai (#1924)

Update debugger in index to reflect in docs.tvm.ai under the Design and Developer Guide

docs/dev/index.rst

@@ -8,6 +8,7 @@ In this part of documentation, we share the rationale for the specific choices m
    :maxdepth: 2
 
    runtime
+   debugger
    nnvm_json_spec
    nnvm_overview
    hybrid_script