docs: add .readthedocs.yaml configuration file

Read the Docs has started requiring a .readthedocs.yaml configuration
file to be present in the repository in order to build the
documentation.

https://blog.readthedocs.com/migrate-configuration-v2/

According to the timeline we have until 2023-09-25 to migrate to the new
configuration file, but we've already had some builds fail during their
48-hour brownout on 2023-09-04.

.readthedocs.yaml

+version: 2
+
+build:
+  os: ubuntu-22.04
+  tools:
+    python: "3.11"
+
+sphinx:
+  configuration: docs/source/conf.py
+
+python:
+  install:
+    - method: pip
+      path: .
+    - requirements: docs/requirements.txt

docs/requirements.txt

+sphinx==7.2.5
+sphinx_rtd_theme==1.3.0