ci: update readthedocs build

taken from
readthedocs/readthedocs.org#4912 (comment)

Author: jtpavlock

.github/workflows/preview_docs.yml

@@ -15,4 +15,4 @@ jobs:
     steps:
       - uses: readthedocs/actions/preview@v1
         with:
-          project-slug: "mrmoe"
+          project-slug: "moe-transcode"

.readthedocs.yml

@@ -6,12 +6,12 @@ version: 2
 build:
   os: ubuntu-22.04
   tools:
-    python: "3.10"
+    python: "3.12"
   jobs:
+    post_create_environment:
+      - python -m pip install poetry
     post_install:
-      - pip install poetry
-      - poetry config virtualenvs.create false
-      - poetry install --without lint,test
+      - VIRTUAL_ENV=$READTHEDOCS_VIRTUALENV_PATH poetry install --with docs
 
 sphinx:
   configuration: docs/conf.py