[MLGO]{Github] Add MLGO docs page to the MLGO PR subscriber group

This ensures that the MLGO PR subscriber team gets pinged if the document gets
modified which makes sense given it is MLGO specific.

.github/new-prs-labeler.yml

@@ -702,6 +702,7 @@ mlgo:
   - llvm/unittests/CodeGen/ML*
   - llvm/test/CodeGen/MLRegAlloc/**
   - llvm/utils/mlgo-utils/**
+  - llvm/docs/MLGO.rst
 
 tools:llvm-exegesis:
   - llvm/tools/llvm-exegesis/**