ReadTheDocs: update environment

Old LuaLaTeX in the default environment is causing us problems

.readthedocs.yml

@@ -4,6 +4,17 @@
 # Required
 version: 2
 
+build:
+  os: ubuntu-22.04
+  tools:
+    python: "3.11"
+    nodejs: "19"
+  apt_packages:
+    - librsvg2-bin
+  jobs:
+    post_create_environment:
+      - npm install -g @mermaid-js/mermaid-cli
+
 # Build documentation in the docs/ directory with Sphinx
 sphinx:
   builder: dirhtml
@@ -15,6 +26,5 @@ formats:
 
 # Optionally set the version of Python and requirements required to build your docs
 python:
-  version: 3.8
   install:
     - requirements: doc/sphinx/requirements.txt