allow skipping RTD builds (#7470)

* configure rtd to skip builds if `[skip-rtd]` is found in the subject

* properly cancel the build early

* try if it actually works [skip-rtd]

Author: keewis

.readthedocs.yaml

@@ -4,6 +4,9 @@ build:
   os: ubuntu-20.04
   tools:
     python: mambaforge-4.10
+  jobs:
+    post_checkout:
+      - (git --no-pager log --pretty="tformat:%s" -1 | grep -viq "[skip-rtd]") || exit 183
 
 conda:
   environment: ci/requirements/doc.yml