Run tests, documentation and frontend build also on pull_request

.github/workflows/documentation.yml

@@ -1,7 +1,7 @@
 name: documentation
 
 # always run documentation workflow to ensure the documentation can still be built
-on: push
+on: [push, pull_request]
 
 # possibly add a publish job for automatic read-the-docs publishing
 jobs:

.github/workflows/frontend.yml

@@ -1,7 +1,7 @@
 name: frontend
 
 # always run frontend workflow to see if it builds and tests run fine
-on: push
+on: [push, pull_request]
 
 jobs:
   frontend:

.github/workflows/test.yml

@@ -1,7 +1,7 @@
 name: backend-tests
 
 # always run tests workflow
-on: push
+on: [push, pull_request]
 
 jobs:
   tests: