docs: Divorcing tools for the main and docs build

Change-Id: Ica15c40b54cde1cf0ae365ed80c38d2f086f6b2f
Signed-off-by: Anton Komlev <anton.komlev@arm.com>
diff --git a/docs/technical_references/instructions/documentation_generation.rst b/docs/technical_references/instructions/documentation_generation.rst
index 72e7e55..b8bd9ca 100644
--- a/docs/technical_references/instructions/documentation_generation.rst
+++ b/docs/technical_references/instructions/documentation_generation.rst
@@ -39,6 +39,11 @@
             # For PDF generation
             sudo apt-get install -y doxygen-latex
 
+            # Additional Python dependencies for documentation
+            pip3 install --upgrade pip
+            cd trusted-firmware-m
+            pip3 install -r tools/requirements_docs.txt
+
         2. Currently, there are two ways of building TF-M reference manual:
 
             - Using the CMake build system as custom targets
@@ -93,6 +98,11 @@
 
         .. code-block:: bash
 
+            # Additional Python dependencies for documentation
+            pip3 install --upgrade pip
+            cd trusted-firmware-m
+            pip3 install -r tools\requirements_docs.txt
+
             set PLANTUML_JAR_PATH=<plantuml_Path>\plantuml.jar
             set PATH=$PATH;<ARM_DS_PATH>\sw\java\bin