add minimal rtd configuration

.readthedocs.yaml

+version: 2
+
+build:
+  os: ubuntu-22.04
+  tools:
+    python: "3.11"
+python:
+  install:
+  - path: .
+  - requirements: docs/source/requirements.txt

docs/source/requirements.txt

 sphinx>=2.1.0,!=3.1.0
-sphinx_rtd_theme
-sphinx-prompt
+sphinx-rtd-theme>=1.2.2
+sphinx-prompt>=1.5.0
 docutils!=0.18