RtD docs previews: Cancel building PRs if no changes in Doc dir

.readthedocs.yml

@@ -13,6 +13,18 @@ build:
     python: "3"
 
   commands:
+    # https://docs.readthedocs.io/en/stable/build-customization.html#cancel-build-based-on-a-condition
+    #
+    # Cancel building pull requests when there aren't changes in the Doc directory.
+    #
+    # If there are no changes (git diff exits with 0) we force the command to return with 183.
+    # This is a special exit code on Read the Docs that will cancel the build immediately.
+    - |
+      if [[ "$READTHEDOCS_VERSION_TYPE" = "external" ]] && git diff --quiet origin/main -- Doc/ .readthedocs.yml; then
+        echo "No changes to Doc/ - exiting the build.";
+        exit 183;
+      fi
+
     - make -C Doc venv html
     - mkdir _readthedocs
     - mv Doc/build/html _readthedocs/html