Add a guide for converting an existing project

[rmartin16]

What is the problem or limitation you are having?

With the introduction of Briefcase's Convert command, it is now even easier to support adding Briefcase to existing projects. However, the conversion process makes assumptions and puts requirements on the state of the project to be converted; so, when errors are encountered during the conversion, it may not be obvious how to handle them.

Especially with support for console apps, it seems reasonable that more devs will approach Briefcase as a post-hoc packaging solution for apps that only run in the console.

Describe the solution you'd like

Add a how-to guide for converting an existing project to Briefcase. While I think the guide should help readers to understand how a Briefcase project is structured, I also think it should be somewhat of a checklist to help make sure the conversion will be optimal.

Describe alternatives you've considered

n/a

Additional context

Related:

• Allow user to manually choose sources directory during conversion #1897
• Do not override existing pyproject.toml keys during conversion #1898
• Include all Briefcase tables in pyproject.toml during conversion #1899
• Ask user if the app is a console app during conversion #1900
• Docs: Add Building a CLI Application Tutorial #1958