add .readthedocs.yaml to fix build

readthedocs.org starts refusing builds with missing or old config files:

| Error
| The configuration file required to build documentation is missing from
| your project. Add a configuration file to your project to make it build
| successfully. Read more at
| https://docs.readthedocs.io/en/stable/config-file/v2.html

First of all, this just adds the basic template suggested under the
above-mentioned link.

Signed-off-by: Enrico Joerns <[email protected]>

.readthedocs.yaml

@@ -0,0 +1,9 @@
+version: 2
+
+build:
+  os: ubuntu-22.04
+  tools:
+    python: "3.11"
+
+sphinx:
+  configuration: docs/conf.py