doc: update with CMake instructions

   https://www.zephyrproject.org/doc/getting_started/getting_started.html

The examples below use a Linux host environment for Zephyr development.

You should be familiar with common developer tools such as Git and Make, and

You should be familiar with common developer tools such as Git and CMake, and

platforms such as GitHub.

If you haven't already done so, you'll need to create a (free) GitHub account

local Zephyr source working directory::

    $ source zephyr-env.sh

    $ make host-tools

    $ export PREBUILT_HOST_TOOLS=${ZEPHYR_BASE}/bin

    $ export USE_CCACHE=1

    $ ./scripts/sanitycheck

    $ ./scripts/sanitycheck --ccache

The above will execute the basic sanitycheck script, which will run various

kernel tests using the QEMU emulator.  It will also do some build tests on

various samples with advanced features that can't run in QEMU.

We highly recommend you run these tests locally to avoid any CI failures.

Using CCACHE and pre-built host tools is optional, however it speeds up the

execution time considerably.

We highly recommend you run these tests locally to avoid any CI

failures. Enabling ccache with --ccache is optional, but it will speed

up the execution time considerably.

Coding Style

   $ cd ~/zephyr

   $ source zephyr-env.sh

   $ cd doc

   $ make htmldocs

Depending on your development system, it will take about 15 minutes to