conf.py: Include platuml extension

Some subprojects, for example meta-ohos, use this extension for uml
diagrams.

Signed-off-by: Andrei Gherzan <andrei.gherzan@huawei.com>

conf.py

@@ -33,7 +33,7 @@ release = version
 # Add any Sphinx extension module names here, as strings. They can be
 # extensions coming with Sphinx (named 'sphinx.ext.*') or your custom
 # ones.
-extensions = ['sphinx_tabs.tabs']
+extensions = ['sphinx_tabs.tabs', 'sphinxcontrib.plantuml']
 
 # Add any paths that contain templates here, relative to this directory.
 templates_path = ['_templates']