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.

For the new configuration style for read the docs, one of the ways to
pass extra configuration for python is having a requirements file.

.readthedocs.yaml

@@ -1,11 +1,18 @@
 # Required by Read The Docs
 version: 2
 
-formats: all
+build:
+  os: ubuntu-22.04
+  tools:
+    python: "3.11"
 
 python:
-  version: "3.8"
+  install:
+    - requirements: doc/userguide/requirements.txt
+
+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

doc/userguide/requirements.txt

@@ -0,0 +1 @@
+sphinx_rtd_theme