Docs: update build customization Poetry example

[sethfischer]

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

---

📚 Documentation previews 📚

• User's documentation (docs): https://docs--9879.org.readthedocs.build/en/9879/

• Developer's documentation (dev): https://dev--9879.org.readthedocs.build/en/9879/

[benjaoming]

Thanks for fixing this! 💯 for noticing the "Docs:" preview on the documentation PRs ❤️

[benjaoming]

Note: The build failure in checks is related to blacken-docs. It's because we are not using correct Python syntax in a code-block:: Python -- it's a change that overlapped with the introduction of the blacken-docs pre-commit hook.