[Starlingx-discuss] Build guide need be updated with layered build

Hi Doc/build team, The build guide wiki [0] is out of date since latest code has been switched to layered build. There is some major change for the step, such as you need sync code first, then download srpm/tarball. Please help update the wiki, otherwise new guy will be blocked for the build environment setup. BTW, before that, we could refer the commit message in [1] to figure what we need to change to setup build environment. [0]: https://docs.starlingx.io/developer_resources/build_guide.html#development-e... [1]: https://opendev.org/starlingx/tools/commit/d51f8050cd20926c20d28c67d8967e807... Best Regards Shuicheng