ci: use ReadtheDocs PR preview instead of building docs in github action

References:
https://docs.readthedocs.io/en/stable/pull-requests.html
https://docs.readthedocs.io/en/stable/config-file/v2.html#sphinx-fail-on-warning

Author: haote.yang

.github/workflows/build_docs.yaml

@@ -0,0 +1,17 @@
+# .readthedocs.yaml
+# Read the Docs configuration file
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+
+---
+# Required
+version: 2
+
+# Build documentation in the docs/ directory with Sphinx
+sphinx:
+  fail_on_warning: true
+
+# Optionally set the version of Python and requirements required to build your docs
+python:
+  version: 3.8
+  install:
+    - requirements: requirements.txt