fix: add build.os key in readthedocs.yaml

As of October 16, 2023, the build.os key of the readthedocs.yaml configuration file is a required part of the configuration. Builds after this date without this key will fail. Please see https://blog.readthedocs.com/use-build-os-config/.

This commit adds a build.os key to fix a failing build.

.readthedocs.yaml

@@ -9,8 +9,14 @@ version: 2
 sphinx:
    configuration: docs/api/source/conf.py
 
-# Optionally set the version of Python and requirements required to build your docs
+# Set the version of Python and other tools you might need
+build:
+  os: "ubuntu-22.04"
+  tools:
+    python: "3.8"
+
+# We recommend specifying your dependencies to enable reproducible builds:
+# https://docs.readthedocs.io/en/stable/guides/reproducible-builds.html
 python:
-   version: "3.8"
    install:
    - requirements: requirements/doc.txt