doc: Add instructions for installing Python dependencies

The Python dependencies specific to building the documentation were
recently removed from the common requirements. Point users to the
documentation generation instructions and provide details on installing
the doc specific dependencies.

Signed-off-by: Keith Short <keithshort@google.com>

doc/contribute/documentation/generation.rst

@@ -93,6 +93,13 @@ as described below:
 
    .. group-tab:: Linux
 
+      Common to all Linux installations, install the Python dependencies
+      required to build the documentation:
+
+      .. code-block:: console
+
+         pip install -r ~/zephyrproject/zephyr/doc/requirements.txt
+
       On Ubuntu Linux:
 
       .. code-block:: console
@@ -122,6 +129,12 @@ as described below:
 
    .. group-tab:: macOS
 
+      Install the Python dependencies required to build the documentation:
+
+      .. code-block:: console
+
+         pip install -r ~/zephyrproject/zephyr/doc/requirements.txt
+
       Use ``brew`` and ``tlmgr`` to install the tools:
 
       .. code-block:: console
@@ -132,6 +145,12 @@ as described below:
 
    .. group-tab:: Windows
 
+      Install the Python dependencies required to build the documentation:
+
+      .. code-block:: console
+
+         pip install -r %HOMEPATH$\zephyrproject\zephyr\doc\requirements.txt
+
       Open a ``cmd.exe`` window as **Administrator** and run the following command:
 
       .. code-block:: console