add readthedocs configuration

.readthedocs.requirements.txt

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+sphinx<4.4.0

.readthedocs.yaml

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+# .readthedocs.yaml
+# Read the Docs configuration file
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+
+version: 2
+
+build:
+  os: ubuntu-20.04
+  tools:
+    python: "3"
+
+sphinx:
+   configuration: conf.py
+
+python:
+   install:
+   - requirements: .readthedocs.requirements.txt