docs: isolate Poetry and Cookiecutter installations

[whg517]

Summary

• replace direct and sudo-based Poetry installation guidance with isolated pipx installs across all three relevant documents
• clarify that the Poetry tool environment must remain separate from both system Python and project environments
• install Cookiecutter through the same pipx workflow and update its documentation link

Validation

• markdownlint-cli2 0.23.2: 30 Markdown files, 0 issues
• MkDocs strict build in a clean Python 3.12 environment
• verified all newly referenced official documentation URLs return HTTP 200
• confirmed the edited guidance contains no sudo pip or direct pip install command for Poetry
• git diff --check

References

• https://python-poetry.org/docs/#installation
• https://pipx.pypa.io/stable/how-to/install-pipx.html

Closes #39