Make pypi & docs-preview actions optional via copier config

[dalito]

Both actions are not required in every project. We should ask if they should be added in the project setup questions.

• pypi_publish.yaml (default: "no") is only needed if the action should be published on PyPI which only some LinkML do.
• test_pages_build.yaml (default: "no") builds a documentation preview for pull requests. However, this does not work for contributions from forks. Since "contributing-from-forks" is the default contribution process on GitHub, this action is not generally useful.

With copier the questions can be answered again as part of an update, which allows to activate the actions later when needed.