[Github] Run documentation test build on change to the workflow

This makes it so that the documentation test build workflow will run
when the workflow itself is changed. This makes it much easier to test
inside of PRs that modify the workflow.

Author: boomanaiden154

.github/workflows/docs.yml

@@ -28,6 +28,7 @@ on:
       - 'polly/docs/**'
       - 'flang/docs/**'
       - 'flang/include/flang/Optimizer/Dialect/FIROps.td'
+      - '.github/workflows/docs.yml'
   pull_request:
     paths:
       - 'llvm/docs/**'
@@ -45,6 +46,7 @@ on:
       - 'polly/docs/**'
       - 'flang/docs/**'
       - 'flang/include/flang/Optimizer/Dialect/FIROps.td'
+      - '.github/workflows/docs.yml'
 
 jobs:
   check-docs-build: