Restrict running ReST lint on push to branches develop and master

Author: RKrahl

.github/workflows/rst-lint.yaml

@@ -1,5 +1,10 @@
 name: Check ReST input files
-on: [push, pull_request]
+on:
+  push:
+    branches:
+      - develop
+      - master
+  pull_request:
 jobs:
   doc8:
     runs-on: ubuntu-latest