build: Use recommended Read the Docs setup

There's still been some failures [2], so I've updated it to follow
their recommendation [1].

[1] https://docs.readthedocs.io/en/stable/build-customization.html#install-dependencies-with-poetry
[2]
```
Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/mlserver/envs/1761/lib/python3.10/site-packages/sphinx/config.py", line 509, in eval_config_file
    exec(code, namespace)  # NoQA: S102
  File "/home/docs/checkouts/readthedocs.org/user_builds/mlserver/checkouts/1761/docs/conf.py", line 19, in <module>
    import sphinx_material
ModuleNotFoundError: No module named 'sphinx_material'
```

.readthedocs.yaml

@@ -24,8 +24,9 @@ build:
       # Install poetry
       # https://python-poetry.org/docs/#installing-manually
       - pip install poetry==1.8.1
-      # Tell poetry to not use a virtual environment
-      - poetry config virtualenvs.create false
+    post_install:
       # Install dependencies with 'docs' dependency group
       # https://python-poetry.org/docs/managing-dependencies/#dependency-groups
-      - poetry install -vvv --without dev --with docs --with all-runtimes --with all-runtimes-dev
+      # 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 -vvv --without dev --with docs --with all-runtimes --with all-runtimes-dev