docs: add read the docs configuration file

Details from https://docs.readthedocs.io/en/stable/tutorial/index.html
and https://mastodon.social/@hynek/109849829588314515

Addresses #49

Author: dhellmann

.readthedocs.yaml

@@ -0,0 +1,12 @@
+version: 2
+
+build:
+  os: "ubuntu-20.04"
+  tools:
+    python: "3.11"
+
+python:
+  # Install our python package before building the docs
+  install:
+    - method: pip
+      path: .