Limit GitHub Actions docs and test workflows depending on changed paths (#6786)

This should result in the docs workflow only running if files in the 
`docs/` folder have changed, and only run the test workflow if files NOT 
in the `docs/` folder have changed.

This commit itself should therefore, I think, only trigger the test 
workflow. I will follow up with test commits for the other scenarios 
(docs-only and both).

.github/workflows/docs.yml

@@ -2,7 +2,12 @@ name: Wagtail Docs
 
 on:
   push:
+    paths:
+    - 'docs/**'
   pull_request:
+    paths:
+    - 'docs/**'
+
 
 jobs:
   build-docs:

.github/workflows/test.yml

@@ -2,7 +2,11 @@ name: Wagtail CI
 
 on:
   push:
+    paths-ignore:
+    - 'docs/**'
   pull_request:
+    paths-ignore:
+    - 'docs/**'
 
 
 # Our test suite should cover: