Fixes#35373.
This changes the example from "run an arbitrary process" to "copy a file". This should make it a bit easier to follow, require less background knowledge to understand, and not use any platform-specific commands that won't work for Windows users.
The most glaring issue with this change is that this doc does not explictly specify how to build and run a builder. I've updated some of the files to hint at this a bit more (such as the `"implementation": "./dist/my-builder.js"`), but another pass is required to figure out the best way to compile a builder and how we want to structure this example to best communicate that.
PR Close#42371
Previously, the examples in the `cli-builder` guide were hard-coded.
This made it impossible to test them and verify they are correct.
This commit fixes this by converting them into a proper mini-app. In a
subsequent commit, tests will be added to verify that the source code
works as expected (and guard against regressions).
Fixes#34314
PR Close#34362
