doc: nrf52_bsim: Update instructions on how to get BabbleSim

:ref:`POSIX Arch dependencies<posix_arch_deps>`.

To target this board you also need to have `BabbleSim`_ compiled in your system.

If you do not have it yet, in `its web page <https://BabbleSim.github.io>`_

you can find instructions on how to

`fetch <https://babblesim.github.io/fetching.html>`_ and

`build <https://babblesim.github.io/building.html>`_ it.

In short, you can do:

If you do not have it yet, the easiest way to get it, is to enable the babblesim group

in your local west configuration, running west update, and building the simulator:

.. code-block:: console

   mkdir -p ${HOME}/bsim && cd ${HOME}/bsim

   curl https://storage.googleapis.com/git-repo-downloads/repo > ./repo  && chmod a+x ./repo

   ./repo init -u https://github.com/BabbleSim/manifest.git -m everything.xml -b master

   ./repo sync

   west config manifest.group-filter -- +babblesim

   west update

   cd ${ZEPHYR_BASE}/../tools/bsim

   make everything -j 8

Define two environment variables to point to your BabbleSim

.. note::

   If you need more BabbleSim components, or more up to date versions,

   you can check the `BabbleSim web page <https://BabbleSim.github.io>`_

   for instructions on how to

   `fetch <https://babblesim.github.io/fetching.html>`_ and

   `build <https://babblesim.github.io/building.html>`_ it.

You will now need to define two environment variables to point to your BabbleSim

installation, ``BSIM_OUT_PATH`` and ``BSIM_COMPONENTS_PATH``.

If you followed the previous steps, you can just do:

.. code-block:: console

   export BSIM_OUT_PATH=${HOME}/bsim/

   export BSIM_COMPONENTS_PATH=${HOME}/bsim/components/

   export BSIM_OUT_PATH=${ZEPHYR_BASE}/../tools/bsim

   export BSIM_COMPONENTS_PATH=${BSIM_OUT_PATH}/components/

.. note::