github: backport suppress running CI only for documentation (#4608)

For updating documentation or release task, running GitHub Actions
is waste of resource.

https://docs.github.com/en/actions/writing-workflows/workflow-syntax-for-github-actions#onpushpull_requestpull_request_targetpathspaths-ignore

Signed-off-by: Kentaro Hayashi <[email protected]>

Author: kenhys

.github/workflows/test.yml

@@ -3,8 +3,14 @@ name: Test
 on:
   push:
     branches: [v1.16]
+    paths-ignore:
+      - '*.md'
+      - 'lib/fluent/version.rb'
   pull_request:
     branches: [v1.16]
+    paths-ignore:
+      - '*.md'
+      - 'lib/fluent/version.rb'
 
 jobs:
   test: