Update RTD poetry install instructions

.readthedocs.yaml

@@ -11,10 +11,16 @@ build:
   tools:
     python: "3.12"
   jobs:
-    post_install:
+    post_create_environment:
+      # Install poetry
+      # https://python-poetry.org/docs/#installing-manually
       - pip install poetry==1.8.3
-      - poetry config virtualenvs.create false
-      - poetry install --with doc
+    post_install:
+      # Install dependencies with 'docs' dependency group
+      # https://python-poetry.org/docs/managing-dependencies/#dependency-groups
+      # VIRTUAL_ENV needs to be set manually for now.
+      # See https://github.com/readthedocs/readthedocs.org/pull/11152/
+      - VIRTUAL_ENV=$READTHEDOCS_VIRTUALENV_PATH poetry install --with doc
 
 # Build documentation in the "docs/" directory with Sphinx
 sphinx: