Document local build process

As per #5286, cloud building is not the preferred method for doing C++ development. However, the documentation does not reflect this anywhere.

For example, documentation for the pxt command only mentions the --cloud command, and indicates that this forces builds to happen "in the cloud". This implies that the default is to not do that.

Indeed, projects (such as https://github.com/microsoft/pxt-chibitronics#setup) indicate that a local toolchain is required to build, and other projects mention the invalid -yt flag (https://github.com/Microsoft/pxt-calliope#running).

There are a few issues here:

1. The --localbuild flag should be documented
2. It should be mentioned that --cloud is default and implied
3. Nonexistent flags such as -yt should be removed from documentation
4. It should be mentioned that, when doing C++ development, a local build environment is preferred.