docs/documentation: Mention installation of PlantUML

Include information about installing PlantUML for the build process now
that it has been included for UML diagram rendering.

Signed-off-by: Matteo Golin <matteo.golin@gmail.com>

Documentation/contributing/documentation.rst

@@ -28,7 +28,15 @@ To render the Documentation locally, you should clone the NuttX main repository
       $ # activate the virtual environment
       $ pipenv shell
 
-  2. Build documentation:
+  2. `Install the PlantUML tool <https://www.javiljoen.net/n/installing-plantuml.html>`_
+     (used to render UML diagrams) and ensure it is on your ``PATH``.
+
+     .. code:: console
+
+        $ sudo apt install plantuml
+        $ plantuml -version
+
+  3. Build documentation:
 
     .. code-block:: console