core: improve entrypoint API / docs

General exploration plan:

Eliminate CoreApp in our build.zig logic.
Eliminate the old core API and make it 100% a module.
Everywhere:
a. Show people how to set up the module system using their own main entrypoint first. i.e. 'what people expect' but is platform-specific.
b. Show them how they could use a Mach-provided entrypoint package instead, which is actually cross-platform.
Eliminate App in our build.zig logic.
Make mach-core's API just a Mach module
Polish our module system architecture enough that mach-core's API could be just a Mach module: module system revamp #1182

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