[workflow] Allow creation of PRs in the docs subdir

Author: tru

.github/workflows/repo-lockdown.yml

@@ -15,6 +15,7 @@ on:
       - 'clang/unitests/Format/**'
       - 'clang/tools/clang-format/**'
       - 'llvm/utils/git/**'
+      - 'llvm/docs/**'
       - '*/foo'
 
 permissions: