Migrate .readthedocs.yml to use build.os (#1258)

Since build.image is being deprecated[0], this changes the config
to use the new format.

[0]: https://blog.readthedocs.com/use-build-os-config/

.readthedocs.yml

@@ -1,12 +1,9 @@
 version: 2
 
 build:
-  image: "7.0"
-
-python:
-  version: "3"
-  install:
-    - method: pip
-      path: .
-      extra_requirements:
-        - docs
+  os: "ubuntu-22.04"
+  tools:
+    python: "3"
+  jobs:
+    post_install:
+      - pip install .[docs]