docs: enable RTD builds (#4258)

.readthedocs.yaml

@@ -0,0 +1,31 @@
+# .readthedocs.yaml
+# Read the Docs configuration file
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+---
+# Required
+version: 2
+
+build:
+  os: ubuntu-20.04
+  tools:
+    python: "3.8"
+
+# Build documentation in the docs/ directory with Sphinx
+sphinx:
+  configuration: docs/conf.py
+  builder: html
+  fail_on_warning: true
+
+# Build documentation with MkDocs
+# mkdocs:
+#  configuration: mkdocs.yml
+
+# Optionally build your docs in additional formats such as PDF
+#formats:
+#  - pdf
+
+# Optionally set the version of Python
+# and requirements required to build your docs
+python:
+  install:
+    - requirements: docs/.sphinx/requirements.txt