Update the document to instruct the user to build, publish, and install a customized commitizen package instead of installing with edit-mode

[bearomorphism]

We need to improve this in the following PR. We can keep it as it's likely the original content. It's installing this package in edit mode. I think the right way is to publish the package somewhere (e.g., PyPI) and install that package

Originally posted by @Lee-W in https://github.com/commitizen-tools/commitizen/pull/1657#discussion_r2605135967