doc: extensions: Refactor cmake build instructions generation

In order to simplify the current implementation and to prepare for a
change where we default to west as the standard build tool, partially
refactor the generation of cmake-based build instructions:

- Introduce `cd-into` as boolean flag that controls whether build
instructions are generated from the current working directory or from
inside the actual app folder. In the case of cmake, this includes
changing into the build folder to run ninja or make.

- Default to building from the current working directory, in the case of
cmake using the `-B` flag to create the build folder.

- Remove the usage of ZEPHYR_BASE with the `zephyr-app` option.
The option itslef is kept fsince it has semantic value, and a comment is
added when used.

- Consolidate the _generate_make and _generate_ninja functions into a
common code block inside _generate_cmake.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>