Cover "Bundled Applications" as an additional topic

We currently don't cover the question of "all inclusive" applications at all, where the whole of the application is bundled up into a single file. While that's beyond the scope of PyPA's responsibilities from a development perspective, we can still provide a useful index pointing folks towards the tools and communities that do address this problem space.

There are two main approaches to cover: those that assume a Python runtime will be available on the target system, and those that bundle the Python runtime as well.

Options that expect a runtime to be available:

• zipapp (standard library): https://docs.python.org/3/library/zipapp.html
• pex: https://pex.readthedocs.io/en/stable/

Options that bundle the runtime as well:

• PyInstaller (Windows, Mac OS X, Linux, *nix): http://www.pyinstaller.org/
• cx_Freeze (Windows, Mac OS X, *nix: https://cx-freeze.readthedocs.io/en/latest/
• py2exe (Windows): http://www.py2exe.org/
• py2app (Mac OS X): https://pythonhosted.org/py2app/
• briefcase (iOS, tvOS, macOS, Android): https://briefcase.readthedocs.io/en/latest/

Start by locating the packaging guide page or navigation area where application distribution topics are documented. Add coverage for bundled applications, separating tools that require an existing Python runtime from those that bundle one, and include the listed projects with their supported platforms. Done means readers can use the page as an index to these tools and communities.