Docs: update build customization Poetry example (#9879)

Move `poetry install` from `post_create_environment` to `post_install`.

This ensures the Sphinx version installed from `poetry.lock` is used to build
the docs, rather than the default version of Sphinx.

Closes: #9869

Author: sethfischer

docs/user/build-customization.rst

@@ -330,6 +330,7 @@ Take a look at the following example:
          - pip install poetry
          # 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 --with docs