Update workflows to skip tests for only .md files (#371)

When updating documentation Markdown files, long-running tests still run
unnecessarily. With these changes, it will skip running test workflows
when PRs only containing changes to such files are opened.

Co-authored-by: xermicus <cyrill@parity.io>

.github/workflows/test.yml

@@ -2,9 +2,13 @@ name: Test
 on:
   push:
     branches: ["main"]
+    paths-ignore:
+      - "**.md"
   pull_request:
     branches: ["main"]
     types: [opened, synchronize]
+    paths-ignore:
+      - "**.md"
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}