docs: Add readthedocs config and requirements.txt

The default versions used for sphinx and docuilts on
readthedocs are no longer compatible. Explicitly list
the package versions that should be used when building
the documentation.

.readthedocs.yml

@@ -0,0 +1,20 @@
+# .readthedocs.yml
+# 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:
+  builder: html
+  configuration: doc/conf.py
+
+# Build the docs in additional formats such as PDF and ePub
+formats: all
+
+# Set the version of Python and requirements required to build your docs
+python:
+   version: 3.7
+   install:
+     - requirements: doc/requirements.txt

doc/requirements.txt

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+sphinx==4.2
+sphinx-rtd-theme==1.0.0
+docutils<0.18