docs: try to unbreak the readthedocs.io build

It was failing with a message:

  Config validation error in build.os. Value os not found.

Apparently, the v2 config file is mandatory, so let's do that.

Change-Id: I267d5314db026de532b2b6644f500d25de08e343

.readthedocs.yml

@@ -1,5 +1,15 @@
+version: 2
 build:
-    image: latest
+  os: ubuntu-22.04
+  tools:
+    python: "3.12"
+
 python:
-    version: 3.8
-requirements_file: docs/requirements.txt
+  install:
+    - method: pip
+      path: .
+      extra_requirements:
+        - docs
+
+sphinx:
+  configuration: docs/conf.py