doc: application: revise app/zephyr link documentation

Application CMakeLists.txt

==========================

Every application must have a :file:`CMakeLists.txt` file. This

file is the entry point, or top level, of the build

system that builds the final :file:`zephyr.elf` image. :file:`zephyr.elf`

contains both the application and the kernel libraries.

Every application must have a :file:`CMakeLists.txt` file. This file is the

entry point, or top level, of the build system. The final :file:`zephyr.elf`

image contains both the application and the kernel libraries.

This section describes some of what you can do in your :file:`CMakeLists.txt`.

Make sure to follow these steps in order.