docs: adjust readthedocs config to new options

Our documentation was failing to build, seems connected to the new way
of indicating build options (cf
https://readthedocs.org/projects/suricata/builds/22112658/,
https://docs.readthedocs.io/en/stable/config-file/v2.html#build,
and https://docs.readthedocs.io/en/stable/config-file/v2.html#build-os).

Added the build.os required new field, and adjusted the way python
version is passed.

.readthedocs.yaml

@@ -1,11 +1,21 @@
 # Required by Read The Docs
 version: 2
 
-formats: all
+build:
+  os: ubuntu-22.04
+  tools:
+    python: "3.8"
 
 python:
-  version: "3.8"
+  install:
+    - method: pip
+      path: .
+      extra_requirements:
+        - sphinx-rtd-theme
+
+sphinx:
+  builder: html
+  configuration: doc/userguide/conf.py
+  fail_on_warning: false
 
-  # Use an empty install section to avoid RTD from picking up a non-python
-  # requirements.txt file.
-  install: []
+formats: all