docs: move the config file

RTD can sometimes be particular about the suffix.  So update to `.yaml`
instead of `.yml`.

Also update the config to match the latest valid RTD config for build.os

.readthedocs.yml → .readthedocs.yaml

@@ -2,15 +2,26 @@
 # Read the Docs configuration file
 # See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
 
-# Required
+# Required: the version of this file's schema.
 version: 2
 
 # Build documentation in the docs/ directory with Sphinx
 sphinx:
   configuration: docs/source/conf.py
   fail_on_warning: true
 
+# Optionally build your docs in additional formats such as PDF
+# formats:
+#   - pdf
+#   - epub
+
+# Set the version of python needed to build these docs.
+build:
+  os: "ubuntu-22.04"
+  tools:
+    python: "3.8"
+
+# Optionally set the requirements required to build your docs
 python:
-  version: 3.8
   install:
     - requirements: requirements/docs.txt