Configure Read the Docs

readthedocs.org build every commit pushed on `master`. However
builds have been failing for the past two months, because they
now require a config file. I assumed that config file was added,
but I was wrong. So I added it.

.readthedocs.yaml

@@ -0,0 +1,12 @@
+# Read the Docs configuration file for MkDocs projects
+# See https://docs.readthedocs.io/en/stable/config-file/v2.html
+
+version: 2
+
+build:
+    os: ubuntu-22.04
+    tools:
+        python: 3
+
+mkdocs:
+    configuration: mkdocs.yml