doc: Consolidate Python and pip documentation

The documentation that describes how Python and pip interact with the OS
when installing packages used to be under a common section, and was
moved to the west bootstrap one later on. Since this information is
required early on (for example on Linux when installing CMake via pip3),
move the info to its own section and link to it from others.

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

doc/getting_started/index.rst

@@ -15,6 +15,33 @@ environment, then build and run a sample application.
 Install Host Dependencies
 *************************
 
+.. _python-pip:
+
+Python and pip
+==============
+
+Python 3 and its package manager, pip\ [#pip]_, are used extensively by Zephyr
+to install and run scripts that are required to compile and run Zephyr
+applications.
+
+Depending on your operating system, you may or may not need to provide the
+``--user`` flag to the ``pip3`` command when installing new packages. This is
+documented throughout the instructions.
+See `Installing Packages`_ in the Python Packaging User Guide for more
+information about pip\ [#pip]_, including this `information on -\\-user`_.
+
+- On Linux, make sure ``~/.local/bin`` is on your :envvar:`PATH`
+  :ref:`environment variable <env_vars>`, or programs installed with ``--user``
+  won't be found\ [#linux_user]_.
+
+- On macOS, `Homebrew disables -\\-user`_.
+
+- On Windows, see the Installing Packages information on ``--user`` if you
+  require using this option.
+
+Install the required tools
+===========================
+
 Follow an operating system specific guide, then come back to this page.
 
 .. toctree::
@@ -46,18 +73,7 @@ First, install the ``west`` binary and bootstrapper using ``pip3``:
    # macOS (Terminal) and Windows (cmd.exe)
    pip3 install west
 
-See :ref:`west-install` for additional details on installing west. See
-`Installing Packages`_ in the Python Packaging User Guide for more information
-about pip\ [#pip]_, including this `information on -\\-user`_.
-
-- On Linux, make sure ``~/.local/bin`` is on your :envvar:`PATH`
-  :ref:`environment variable <env_vars>`, or programs installed with ``--user``
-  -- like west -- won't be found\ [#linux_user]_.
-
-- On macOS, `Homebrew disables -\\-user`_.
-
-- On Windows, see the Installing Packages information on ``--user`` if you
-  require using this option.
+See :ref:`west-install` for additional details on installing west.
 
 .. _clone-zephyr:
 

doc/getting_started/installation_linux.rst

@@ -119,7 +119,7 @@ Arch Linux:
 using ``cmake --version``. If you have an older version, there are several ways
 of obtaining a more recent one:
 
-* Use ``pip``:
+* Use ``pip3`` (see :ref:`python-pip` for more details):
 
   .. code-block:: console
 

doc/guides/west/install.rst

@@ -15,7 +15,7 @@ On Windows and macOS::
   pip3 install -U west
 
 .. note::
-   See :ref:`gs_python_deps` for additional clarification on using the
+   See :ref:`python-pip` for additional clarification on using the
    ``--user`` switch.
 
 Afterwards, you can run ``pip3 show -f west`` for information on where the west