Update readthedocs config

Doc builds on readthedocsc.org were still failing because of the missing
`build.os` key. Update the config to conform with the recommendations
from https://docs.readthedocs.io/en/stable/config-file/v2.html

.readthedocs.yaml

+# See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+
 version: 2
+
+build:
+  os: ubuntu-22.04
+  tools:
+    python: "3.12"
+
 mkdocs:
   configuration: mkdocs.yml
+
+# Build all formats
+formats: all