Linkcheck and prepare alpha (#2078)

.readthedocs.yaml

@@ -9,6 +9,13 @@ sphinx:
 formats: [htmlzip]
 
 python:
-   version: "3.8"
    install:
    - requirements: docs/.sphinx/requirements.txt
+
+build:
+   os: "ubuntu-20.04"
+   tools:
+      python: "3.8"
+   jobs:
+      pre_build:
+         - python -m sphinx -b linkcheck docs/ _build/linkcheck